Find least recently used mysql table index
Starting with MySQL 5.6, the performance_schema instruments Table I/O, and computes aggregated statistics by table, and by index.

See table performance_schema.table_io_waits_summary_by_index_usage:

http://dev.mysql.com/doc/refman/5.6/en/table-waits-summary-tables.html#table-io-waits-summary-by-index-usage-table

Finding the least recently used index involves time and timestamps.

The performance schema measure counting I/O against an index.  It can be used to find the least often used index, which in practice should be pretty close.

Full example here:

http://sqlfiddle.com/#!9/c129c/4