If you want to drop an unused index, then instead of dropping immediately, you can mark it as invisible, monitor the application behavior, and later drop it. Later, if you need that index, you can mark it as visible, which is very fast compared to dropping and re-adding indexes.
To explain the invisible index, you need to add normal index if not already there. Example:
mysql> ALTER TABLE employees ADD INDEX (last_name);Query OK, 0 rows affected (1.81 sec)Records: 0 Duplicates: 0 Warnings: 0