As the bill S.2658 text currently stands [0], if iMessage has more than 100M monthly active users, then yes. It is hard to find such statistics on a quick search, but the surest way to make sure it applies to iMessage is to call your representative and tell them the threshold should be lowered from 100M to 10M MAU :)
[0] https://www.congress.gov/bill/116th-congress/senate-bill/265...