It is available to all customers[1] with a supported device (plan doesn't matter, but I suspect prepaid customers are more likely to have cheaper, unsupported devices).
Hmmm. Interesting.. When I was on postpaid, I used to get caller ID with something like "Scam Likely", but since switching to pre-paid, I don't get those anymore. Looking at their page about it, it only mentions post-paid.
I can't be sure about Metro since I don't use them, but I've gone from postpaid to pre-paid on T-mobile and I no longer get the the warning/caller ID saying it. Even on their page, it does not mention anything about pre-paid.
Also, their website doesn't have the option for me like it used to, and here's a thread on their site from 2018 that talks about it not being available to pre-paid users:
