Finding indexes that aren't used often or at all
You should enable the user_statistics plugin. After having this active for a while, check out the INDEX_STATISTICS table in the information_schema. The table shows which indexes have been used since last time MariaDB was started, so if they're not listed here then they're good candidates for being purged.

Also use pt-duplicate-key-checker to find duplicated indexes.

All this is nicely written up by Federico Razzoli in Dropping useless MySQL indexes.