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)
The fact that Signal users are still unaware of where their data is going and when should tell you all you need to know about how trustworthy the service is. Not being 100% clear about the risks people take when using software which is promoted for use by people whose freedom and/or lives depend on it being secure is a very bad look for Signal.
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)