Remove unused Indexes MariaDB
Refer to https://mariadb.com/kb/en/user-statistics/
An optional feature of MariaDB collects statistics on usage of indexes (and also tables, users, and clients).
If you enable the userstat plugin (see the documentation link), it records a counter of the rows that were read via each given index. If that number remains 0 for a given index, then it was not used by any query.
Keep in mind these counters are reset to 0 when the MariaDB server restarts, so you don't know if the index was used prior to that.