see here:
https://gitlab.common-lisp.net/asdf/asdf/blob/master/find-system.lisp#L187
CLEANUP-SYSTEM-DEFINITION-SEARCH-FUNCTIONS attempts to remove SYSDEF-PRELOADED-SYSTEM-SEARCH from the hooks, but it is never added. and right under it, SEARCH-FOR-SYSTEM-DEFINITION also assumes that it's supposed to be called explicitly after the hooks.
the reason it's relevant is that this way it's not possible to add a hook as a very last hook to be called (e.g. for quicklisp integration to initiate downloading if a system cannot be found anywhere).