Do you not send thank you cards for birthday and holiday gifts?

Physical thank you cards are pretty dead. I don't even keep track of mailing addresses for a number of my friends (and a couple siblings, come to think of it) - how would I send them a physical card?

Even older relatives - we sent a physical gift a bit ago, but the response/thanks was by text. It just doesn't make sense to send a letter, have it take a week, never know whether it got lost, etc.

The only people I know of that do this are over 60.

No. In person, text, or phone call.