Why is there a 3.8 version?

I can't say why it exists, but it appears to be only a change to the readme file: https://git.drupalcode.org/project/user_activity_log/-/commits/8.x-3.8/?...

It's behind the current release and should just be ignored. Bit it does get accidentally downloaded if you don't type the version constraint.

Not able to delete this tag because it is related to one of the releases. this release is not required but can't delete it. so closing this issue.

I guess the remaining problem is that when you require as drupal/user_activity_log, it gets the 3.8 version by default. So most people will encounter this issue.

Since the system prevents deleting the 3.8 tag, I think the best way to resolve this is to do a 4.0 release with the current 1.10 code.

Sounds like a good solution to me.