User IDs are definitely private info, since they'd allow you to follow a user's behaviour etc.

"user 1234567 viewed post 89564385943"

I can see how that's private info, but would this be:

"client x viewed user 1234567's profile"

If the user id can be joined with other data to give a name or similar then it is considered personally identifying in terms of GDPR.

I am not a lawyer. You should hire an appropriately qualified lawyer to review your data hygiene practices.

Sure, if user 1234567 isn’t particularly popular.