But there’s still no way to know if that number later gets released to another person.
Some carriers do make available lists of recycled numbers, and some telephone information companies aggregate these lists, when I was looking, coverage was sparse though, and questions about reliability and privacy were too big relative to the limited coverage. Sharing of confidential information was an issue too: carriers wanted to provide events only for numbers of interest to a 3rd party service, so the carrier didn't divulge the number of customers leaving their service; the 3rd party service didn't want to provide numbers of interest because it would divulge user count. I may be biased (I was working for a service), but the recycled number list feels less privacy invasive than providing numbers of intetest. The numbering space is small, so you can't meaningfully obscure the numbers, etc. Determining which carrier is responsible for a number is also tricky, of course.
Of course, getting a working incoming number for all countries worldwide that doesn't cost users an arm and a leg to message is not exactly easy.
Note that it's pretty hard to validate a phone number belongs to someone. You could ask their telephone company, but their telephone company shouldn't disclose subscriber information, and even if they did, maybe you get a name, but lots of people have the same name as me, so what does that show?
Commonly, people send a text message with a code or a phone call with a code to demonstrate control (not belonging), but that's almost always automated, and you would need the number to be validated before you could validate it.