`--prune` will delete your local copies of the origin's branches (e.g. `origin/whatever`). But it won't delete your local branches (e.g. `whatever` itself). So PRs that you've worked on or checked out locally will never get deleted.

> Dont most git instances, like github, delete branch after a PR was merged, by default?

By default, I don't think so. And even if the branch is deleted, objects can still be there. I think GitLab has a "Clean stale objects" thing you can trigger, I don't seem to recall ever seeing any "Git Maintenance" UI actions on GitHub so not sure how it works there.

> I think GitLab has a "Clean stale objects" thing you can trigger

This deletes orphaned objects in commits that are unreachable after force-pushes or deleting branches or such, it doesn't delete branches itself.

In Github it needs to be explicitly configured (Settings > General > Delete head branches after merging), Gitlab is the same.

A lot of my developer colleagues don't know how git works, so they have no idea that "I merged the PR" != "I deleted the feature branch". I once had to cleanup a couple repositories that had hundreds of branches spanning back 5+ years.

Nowadays I enforce it as the default project setting.