How are new user IDs calculated automatically by default in Linux?

Question

Basic Linux users are by default identified using some (positive?) numerical ID and at least some years ago there was no "magic" available to keep track of formerly used IDs in case of e.g. user accounts were deleted completely. New users might have got former numerical IDs and such things, simply because the new automatically calculated one was the highest known to the local system of available, not deleted users +1.

Have things changed since then? Might this depend on the distribution or tools used to manage users? Is there anything standardized or in discussion currently?

I'm not interested in things like LDAP or full-blown management software for users, but more low level, really simple tools which are available most likely everywhere. useradd, userdel etc., working on the default files passwd, shadow, group.

Answer 1

No. If a user has been entirely deleted using the standard mechanisms, there is no record of their prior existence. What you want to do is to deactivate users, not delete them, so their user ids are not available for reuse.