Syntax:
DROP FUNCTION [IF EXISTS] function_name

This statement drops the loadable function named function_name. (DROP
FUNCTION is also used to drop stored functions; see [HELP DROP
PROCEDURE].)

DROP FUNCTION is the complement of CREATE FUNCTION. It requires the
DELETE privilege for the mysql system schema because it removes the row
from the mysql.func system table that registers the function.

DROP FUNCTION also removes the function from the Performance Schema
user_defined_functions table that provides runtime information about
installed loadable functions. See
https://dev.mysql.com/doc/refman/8.0/en/performance-schema-user-defined
-functions-table.html.

During the normal startup sequence, the server loads functions
registered in the mysql.func table. Because DROP FUNCTION removes the
mysql.func row for the dropped function, the server does not load the
function during subsequent restarts.

DROP FUNCTION cannot be used to drop a loadable function that is
installed automatically by components or plugins rather than by using
CREATE FUNCTION. Such a function is also dropped automatically, when
the component or plugin that installed it is uninstalled.

URL: https://dev.mysql.com/doc/refman/8.0/en/drop-function-loadable.html

