First, it is necessary to sort out who this letter was written to. It is headed "Hebrews", so it would - in the main - be addressed to Hebrew people, Jewish people in other words, who had become Christians at some point prior to the destruction of the temple and sacrificial system in Jerusalem. Had this letter been written after the catastrophic events of A.D. 70, the sections speaking about the earthly temple would have been very different.
The opening verse is clear that it addresses those people who had the Hebrew prophets as their ancestors, who are called "the fathers" of the people written to. The second verse starts a pattern of speaking about Jesus Christ as the ones those people believe in and follow.
Second, what does "those former days, when after being enlightened, you endured sufferings" mean? Given the approximately 40-year time-span from Jewish people becoming Christians until the destruction of Jerusalem, it would mean at the start of Jewish conversions to Christ ("being enlightened" = being converted to faith in Christ). The book of Acts details the sudden, dramatic and numerically massive rate of such conversions, starting with Jewish people, then spreading out to Gentiles. From that record we learn that the believers "greatly increased" in Jerusalem, and that many Jewish Priests had also been enlightened. This was all before Stephen became the first Christian martyr. (See Acts 6:7.) After Stephen was stoned to death, a great persecution began, and many of the Christians were scattered.
One year into Christianity, the believers had increased from around 1,000 to 10,000. Perhaps by A.D. 60 there were around 100,000. So, when the writer of that letter to Hebrew Christians was penned, the former days of persecution might have had specific reference to that first "batch" of Jewish converts experiencing the first wave of persecution that caused many of them to flee from Jerusalem and its environs. Saul of Tarsus was a primary persecutor of Christians (until he was 'enlightened' by a miraculous vision of Christ while on the road to Damascus to persecute Christians there.)
Third, who were made "a public spectacle"? Once that initial wave of persecution started (spearheaded by Saul of Tarsus), all Christians caught up in it were publicly exposed to vilification and shame, but particularly Jewish people who had their own nation turn on them. Of course, when Saul of Tarsus was converted to faith in Christ, he especially was exposed to public ridicule and attack from his own people. The book of Acts is full of that. Once the synagogues closed to Christians like him, that public spectacle was obvious to all.
Fourth, who became sharers with those so treated? This is where the A.V. is helpful. Verse 34 reads:
"For ye had compassion of me in my bonds, and took joyfully the
spoiling of your goods..." Hebrews 10:34 A.V.
This speaks of Christians who knew Paul (after his conversion), when he was imprisoned, and happily identified with him, despite their goods being taken from them in the persecution they endured, for Christ's name sake.
That includes all converts to Christ in what could be called "the second wave" of conversions, but in the book of Hebrews, that has particular reference to Jewish converts.