DROP FUNCTION UDF

Syntax:
DROP FUNCTION [IF EXISTS] function_name

This statement drops the user-defined function (UDF) 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 UDFs. 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 UDFs 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 UDF that is installed
automatically by server components or plugins rather than by using
CREATE FUNCTION. Such a UDF 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-udf.html