There is no personal pronoun in 2 Thessalonians 2:
7 τὸ γὰρ μυστήριον ἤδη ἐνεργεῖται τῆς ἀνομίας· μόνον ὁ κατέχων ἄρτι ἕως ἐκ μέσου γένηται
Let's look at the genders:
For the mystery [neuter] of lawlessness [feminine] is already at work; only He who now restrains
the [one who]
Article - Nominative Masculine Singular
Strong's 3588: The, the definite article. Including the feminine he, and the neuter to in all their inflections; the definite article; the.
Verb - Present Participle Active - Nominative Masculine Singular
Strong's 2722: From kata and echo; to hold down, in various applications.
Let X be the implied subject.
How do the translators get ‘he’ from X?
We know that X is not referring to the mystery [neuter] or to the lawlessness [feminine]. We also know that X is the masculine subject/actor for the verb restrains or holds down. It is reasonable to assume that X is not a thing or it.
So 'he' is a reasonable translation here.
now restrains will do so until he is taken out of the way.
he is taken
Verb - Aorist Subjunctive Middle - 3rd Person Singular
Strong's 1096: A prolongation and middle voice form of a primary verb; to cause to be, i.e. to become, used with great latitude.
This Greek word is only two Greek words away from the article.
Due to referent proximity, it is probably referring to what the masculine article implies. That's why translators use 'he' here as well.
In any case, a more faithful and conservative rendering is the following:
For the mystery of lawlessness is already at work; but the one who now restrains will do so until it is taken out of the way.
This way, I can avoid the gendered pronouns.
Why Paul didn’t use a pronoun?
Unlike Greek, English articles and verbs do not have gender declensions, so we have to articulate the gender in the form of its pronoun explicitly. However, implied pronouns by the Greek article and verbs were standard practices by the NT writers. They didn't have to write out the pronoun. Paul probably thought that he was obviously talking about a 'he' when he wrote this verse. This bears out among the sentiments
in https://biblehub.com/2_thessalonians/2-7.htm; 20 out of 27 versions use the pronoun 'he'.