All custom mobs now despawn. Including pet special event mobs. Cri. (This means every single Super Turkey and Blizz Ard from the past few years is gone too)
I lost the 2014, 2015, and 2016 versions of Blizz Ard as well as 2015 and 2016 Super Turkeys from my outpost menagerie. Once upon a time I had a 2014 Super Turkey as well but it had already despawned during a previous glitch. Looks like I'll have to come up with a new use for that space.
Dang! I had several Bliz hanging around my wild base that I just hadn't taken the time to hunt down yet.
Maybe he can post back here with his results so that we all know this is something we can do (provided we have proof). I have a screenshot of the 2 I had in my jail cell.