It isn't an optional feature. If you decide not to create a pin one is created for you and your data is still uploaded to the cloud.

See: https://web.archive.org/web/20230616082821/https://old.reddi...

https://signal.org/blog/sealed-sender/ says "By design, it does not store a record of your contacts, social graph, conversation list, location, user avatar, user profile name, group memberships, group titles, or group avatars."

However, today the reality is that signal collects your name, photo/avatar, and your contacts and stores it forever in the cloud.