Are idx_scan statistics reset automatically (default)?
Quote from the manual


  When recovery is performed at server start (e.g. after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset


In other words, the statistics are not reset on a clean shutdown and restart. 

So if your server never crashed, and you never issued a "shutdown immediate" (using `pg_ctl stop -m immediate) then the statistics show the cumulated values since the instance was created (for "global" statistics") or since the database was created (for database specific statistics). 

If you want to detect if there are unused indexes, I would reset the statistics, then take snapshots of them in regular intervals (e.g. every day or every hour depending on your system) and then compare the values over time after e.g. a week of normal operation. 

Note that you can't (shouldn't) remove unique indexes (e.g. a primary key index) that have never been used for reading!