Signal used to never collect data on users, but they've changed that a while ago and now they keep user's name, photo, phone number, and a list of their contacts permanently in the cloud protected from the government by nothing except by a leaky enclave and a pin (https://web.archive.org/web/20250117232443/https://www.vice....)

More recently they've started collected the contents of messages into the cloud too, yet to this very day their privacy policy opens with the lie: "Signal is designed to never collect or store any sensitive information." which hasn't been true for a very very long time. I consider their refusal to update their privacy policy to be a massive dead canary warning people that the service has already been compromised, but feel free to take your chances.

You're able to disable the pin feature to prevent that data from being saved though, so it definitely isn't a requirement.

I'm also not sure where you've read that they collect the contents of messages, because as far as I'm aware they still aren't doing that and I can't find any info online that indicates that they are (other than their secure backup feature that's opt-in only I suppose)