Pogo has re-released 50 previously-retired Mix-n-Match badges and are on sale for 12 gems each until October 9th.
To find these badges easier, sort by "On Sale" and look for the badges marked "Re-released".
https://www.pogo.com/challenges?filters=MNM%2CON_SALE%2CUNOWNED&sort=RELEASE_DATE_REVERSED
So, I suppose these retired badges are something like many professional athletes.... retired for the moment.
Tx for info.
Some of these are worth 300-400 Pogis, so it's worth picking up if you're close!