Apple blackholed my SMSs from certain people for years.
Are you sure it's Apple?
I had a similar problem for years, and it turned out to be AT&T.
I mentioned it to an AT&T rep when I was on the phone correcting a billing error. He was able to do something on AT&T's end, then send a signal that actually rebooted my iPhone remotely (!), and ever since then I get every SMS perfectly fine.
Are you sure it's Apple?
I had a similar problem for years, and it turned out to be AT&T.
I mentioned it to an AT&T rep when I was on the phone correcting a billing error. He was able to do something on AT&T's end, then send a signal that actually rebooted my iPhone remotely (!), and ever since then I get every SMS perfectly fine.