amay077 had a written cleanup: make a working branch and a worktree, then delete both when the work is done. The working branch became main. Cleanup deleted main.

I reran that occupancy check on git 2.53.0. From a sibling branch other, git worktree add wt-main main checked main out in the second tree. git branch -d main printed cannot delete branch 'main' used by worktree at ... and exited 1. After git worktree remove, the same git branch -d main deleted main and left only other. I did not need -D. main was already merged into other.

Git refuses the delete while some worktree still holds the branch. It does not protect the default branch once that worktree is gone. I already wrote about worktrees as identity and as shared repository state. Pin the created name at add-time. Refuse origin/HEAD and the default branch even after the directory is gone.

I would score add_name, occupied, and delete_target separately. A missing worktree is not a reason the name is safe to delete.