When an installed UDF can't be loaded at startup as its .so file can't be found anymore,, the error log will contain warnings about not being able to load the function
but the function can't be removed with DROP FUNCTION:
It can only be removed by deleting the related row in the mysql.func table directly with
DELETE FROM mysql.func WHERE name='...'
It should be able to remove a function with DROP even if it couldn't initialize for whatever reason, including missing .so file