Finding indexes that aren't used often or at all Is there a facility or a recommended practice for determining which indexes are being used and which ones can be "purged"? At the minimum, I'm looking for a log file that over time can be analyzed for which indexes were utilized. From there I can begin researching indexes that were never hit for possible candidates for a clean up.

I'm specifically not interested in running "explain" across our space of queries.
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.