1. The grammar is ambiguous. The key verb 'genesthai' is aorist infinitive. It could be about the future.
In particular in this case, the sentence could be a double ellipsis. Thomas Belsham makes this argument in A Calm Inquiry Into the Scripture Doctrine Concerning the Person of Christ, § 3, pp. 53-55.
As summarized in the article Jesus resorts to a double ellipsis in John 8:58?
"If, instead of interpreting genesthai as absolute (“to be [born]”),
we suppose that genesthai means “to become” (in an elliptical sense to
be determined), the key phrase at John 8:58 becomes:
[Lit. Eng.] before Abraham becomes [ellipsis], I am [ellipsis]
Neither “becomes”, nor “I am” are, reasonably, used in an absolute
sense. So, there may be an ellipsis associated with each verb.
Unpacking the [double ellipsis], we may have:
“Before Abraham becomes [father of a multitude (viz. of nations) – Gen
17:5] I am [the Messiah]”
2. Because the grammar is ambiguous, the argument for time reference is therefore contextual. The main contextual argument for this being about the past is John 8:57, where the hostile Jews ask
"You are not yet fifty years old, and You have seen Abraham?"
It seems the context is about a past event - Abraham.
3. Yet, Jesus regularly ignores questions like these in the Gospel of John and continues with the point He wants to make.
Here are some examples.
John 3, Nicodemus.
3 Jesus replied, “Truly, truly, I tell you, no one can see the kingdom
of God unless he is born again.” 4 “How can a man be born when he is
old?” Nicodemus asked. “Can he enter his mother’s womb a second time
to be born?” 5 Jesus answered, “Truly, truly, I tell you, no one can
enter the kingdom of God unless he is born of water and the Spirit.
You can see here that Nicodemus doesn't understand what Jesus is saying, and Jesus doesn't answer his question (“Can he enter his mother’s womb a second time to be born?”), instead continuing on from his previous point. The 'truly, truly' doesn't apply to Nicodemus' immediately preceding question, but what Jesus has said before that. In other words, the semantic context of John 3:5 isn't 3:4 but 3:3.
John 4, Samaritan woman at the well
10 Jesus answered, “If you knew the gift of God and who is asking you
for a drink, you would have asked Him, and He would have given you
living water.” 11 “Sir,” the woman replied, “You have nothing to draw
with and the well is deep. Where then will You get this living water?
12 Are You greater than our father Jacob, who gave us this well and
drank from it himself, as did his sons and his livestock?” 13 Jesus
said to her, “Everyone who drinks this water will be thirsty again.
14 But whoever drinks the water I give him will never thirst. Indeed,
the water I give him will become in him a fount of water springing up
to eternal life.”
The Samaritan woman doesn't understand what Jesus is saying, and Jesus doesn't answer her question ("Where will you get this living water?"). Instead he continues with the point made before that. The semantic context of John 4:13 is not John 4:12 or 4:11 but John 4:10. Implied, however, is that Jesus is indeed > Jacob.
John 6, Bread of life
47 “Truly, truly, I tell you, he who believes has eternal life. 48 I
am the bread of life. 49 Your fathers ate the manna in the wilderness,
yet they died. 50 This is the bread that comes down from heaven, so
that anyone may eat of it and not die. 51 I am the living bread that
came down from heaven. If anyone eats of this bread, he will live
forever. And this bread, which I will give for the life of the world,
is My flesh.” 52 At this, the Jews began to argue among themselves,
“How can this man give us His flesh to eat?” 53 So Jesus said to them,
“Truly, truly, I tell you, unless you eat the flesh and drink the
blood of the Son of Man, you have no life in you. 54 Whoever eats My
flesh and drinks My blood has eternal life, and I will raise him up at
the last day. 55 For My flesh is real food, and My blood is real drink.
56 Whoever eats My flesh and drinks My blood remains in Me, and I in
him. 57 Just as the living Father sent Me and I live because of the
Father, so also the one who feeds on Me will live because of Me.
58This is the bread that came down from heaven. Unlike your fathers,
who ate the manna and died, the one who eats this bread will live
forever.”
As with Nicodemus and the Samaritan woman, the crowd doesn't understand what Jesus is saying, and Jesus doesn't answer their question ("How can this man give us His flesh to eat?"). Instead, He continues with the point made before that. The semantic context of 6:53 isn't 6:52 but 6:51. Implied in all of this is that Jesus > Moses.
Does the exchange at 8:58 fit this pattern?
Before 8:58, the last point Jesus makes is at 8:56, which is about the future.
"Your father Abraham rejoiced that he would see My day. He saw it and
was glad."
This is referring to the vision Abraham was given in Genesis 22:18
"And through your offspring all nations of the earth will be blessed"
The crowd responds to Jesus' statement at 8:56 with a question showing they don't understand what Jesus is saying (Jesus is not claiming to be a super-old human), at 8:57.
Then the Jews said to Him, “You are not yet fifty years old, and You
have seen Abraham?”
Note also that Jesus never claims to have seen Abraham - that could easily be a mistaken belief by the crowd, again similar to Nicodemus, the Samaritan woman, and the Bread of life crowd's incorrect interpretations of what Jesus is saying, reflected by their questions, in those instances.
Just as with the examples at John 3, John 4, and John 6, it seems reasonable to believe Jesus doesn't answer this question at 8:58, but continues on with his previous point from 8:56. Also, just as with John 4 (> Jacob) and John 6 (> Moses), Jesus also makes a point that He is > Abraham here.
4. John has already given us the key to the 'ego eimi' statement, which is at John 4:26.
John 4:25 is
"The woman said, “I know that Messiah” (called Christ) “is coming.
When He comes, He will explain everything to us.”"
Jesus responds
I who speak to you am He.
The Greek is a bit obscured here by typical English translations, which divide up the 'I' and 'am', but this uses the same phrase as at John 8:58, 'ego eimi', and it is together in the Greek.
"I am (ego eimi)" who? The Christ. There's no debate about the referent of the 'ego eimi' at John 4.
Indeed, much of the Gospel of John concerns Jesus' claim to be the Christ, and the debate is about that. See John 10:24, after John 8, where the question the crowd has is
"How long will you keep us in suspense? If you are the Christ, tell us
plainly."
Although the reader has been told plainly at John 4, most people in John's Gospel don't know this at John 8 (and indeed whether He is the Christ is the central question Caiaphas asks at his trial in the synoptic Gospels, after which He is charged with blasphemy).
5. 8:58 isn't the first use of 'ego eimi' by Jesus in John 8, rather it's the third use. If the 'ego eimi' at 8:58 involves an ellipsis, it falls into a plausible pattern of use of 'ego eimi' in John 8.
At both John 8:24 and 8:28, Jesus says the same thing as at 8:58 ('ego eimi'). Many (perhaps most) translators have no problem translating this at 8:24 and 8:28 as an ellipsis ("I am [he]" or "I am [the one]"). Again, using the key given at John 4:26, where the referent for Jesus' 'ego eimi' statement is clear, i.e., the Christ, it is reasonable to hold the first two uses of 'ego eimi' in John 8 follow the precedent set at John 4:26. If that's right, then 8:58 involving an ellipsis just follows the pattern of Jesus' intended use of 'ego eimi' in John 8 before 8:58.
6. The closest preceding clue for what the 'I am he' or 'I am the one' ('ego eimi') refers to is John 8:12, where Jesus says
"I am the light of the world ('kosmos' = human world, or society)"
This is very similar to John 1:9, which says
"The true Light who gives light to every man was coming into the
world ('kosmos')."
In the prologue, the 'true Light' is contrasted with John the Baptist, who we are told
"He himself was not the Light, but he came to testify about the
Light."
John's testimony in particular is at John 1:34
"I have seen and testified that this is the Son of God."
This is buttressed at John 3:28 where John the Baptist says
"You yourselves can testify that I said, ‘I am not the Christ, but am
sent ahead of Him.’"
So there is a straightforward connection between Jesus being the Light in the prologue and his being the Christ. To listeners at John 8:12, they would not have John 1-3 (and John 4 of course, which gives us the 'ego eimi' key, i.e., "I am the Christ"). For them, the question at John 8:25 (in response to Jesus' first 'ego eimi' in John 8 that is an ellipsis) is
"Who are you?"
and then again at John 8:53
"Who do You claim to be?"
This prompts Jesus' response about Abraham having a vision of Jesus' day (i.e., the coming of the Christ, through whom all nations would be blessed).
Jesus begins his discussion by claiming to be the Christ ('the Light'), but that is not clear to the listeners. He then ends the back and forth by again claiming to be the Christ (at 8:58), but again, not outrightly saying it (which prompts the question at John 10:24, "“How long will You keep us in suspense? If You are the Christ, tell us plainly.”").
So the closest explicit clue Jesus gives as to the referent of 8:58's ellipsis is 8:12, the Light, and in context it is fairly straightforward in the context of the Gospel of John that this is another term for the Christ (but not a term the Jewish audience would have necessarily understood).
7. According to Jesus, the Father is the God of Abraham, and the God of the living, not the dead. Therefore things pertaining to what happens to Abraham can appropriately be talked about in the future, not just the past - Abraham will see Jesus' day.