I find this blue-bar alert unnecessary. Again, for new users it’s probably useful, but I find it adds to the blink-y feeling of the current interface, with the other UI hints that always appear and then disappear when I start typing “Type / for commands” and the constantly-reloading Sidebar.
I think it shows on as few as 3 rems, which is a bit common to delete. Maybe there’s a setting, “alert me if n rems are deleted…” And then the user can choose n, and it’s more than something like 50, by default.