Is there a reasonable alternative, though? Email addresses? Adding a cryptographic challenge to prevent somebody from generating tons of accounts?
As far as phone numbers go, I'm not a big fan of Signal having them, but I definitely prefer not having to give them out! That change is a huge deal to me, as I can now communicate with people without handing them a phone number. And Signal has provided their client and server source code, along with evidence that their servers store absolutely nothing.
Nowadays, the most likely way your Signal data will get leaked is if somebody screenshots it.
Jami doesn't require a phone number, which is p2p. Xmpp (+ Omemo) doesn't require a phone number and it's federated... I mean, if a service is willing to rid of phone numbers, it'll do totally without them.
The challenge of having your device solve a nasty PoW that takes minutes would not deter most people: a timer once is better than evil captchas, phone numbers, etc. I don't understand why they use hCaptcha and not that.
phone numbers for spam prevention are a bandaid for a mediocre solution. the mediocre aspect being that it's totally centralized when it should at least be federated like SimpleX. SimpleX is the ultimate solution to be honest with you, it's federated, fast, extremely private and extremely secure.