new carrier phones and feature and samsung phones in 2023-24

That I can't say. TMobile has been very hands off when it comes to customizing OneUI. ATT is bad for other devices so I wouldn't be surprised if they stripped it.

Unfortunately unless someone else here knows the only way would be to flash U1, start from scratch and test but that's a lot of work for something that may or may not work.
If he went back to carrier firmware he would have Samsung messages RCS
 
But the carrier's are going away with their own RCS and going Google way that's why u see both Samsung/Google collaboration app and Samsung version , Samsung version will most likely ride off into the sun