Merging with "Squash merge" we merge the changes to the main branch, but the commits still exists on the source branch and they're not reachable from the head of the main branch.

This works similar way with git log. If we execute "git log main_branch..branch1" in a local clone of such a repo where we merged branch1 to main_branch with Squash we will still see the list of commits that were merged with squash.

It would be a useful feature not to take into account those commits, which have been merged with Squash