They don't usually remove features as important as iMessage from older iOS versions. I don't believe they push updates to the iPhone 7 and older anymore, so they'd be unable to use iMessage.
I have a 6sPlus, and messages work just fine, and it may not be iOS 17, but I recently-ish ran an update for its OS that Apple deliberately updated (which you just know must have been an important update). You can stop making stuff up now
