It was a bit of a controversy when the change happened:

see https://web.archive.org/web/20210109010728/https://community...

https://www.vice.com/en/article/pkyzek/signal-new-pin-featur...

Note that the "solution" of disabling pins mentioned at the end of the article was later shown to not prevent the collection and storage of user data. It was just giving users a false sense of security. To this day there is no way to opt out of the data collection.