Current HA10 retraining Pool:
Has all old HotM up to and including Miki. I’ll keep an eye out for when Grazul is added to the pool.
As a general FYI, you can check the retraining Pool of heroes available at any time when you’ve built and researched HA10 by tapping the little “info” button.
NOTE: need to have a spare HA action to check the pool tho as you need to open the “retraining” screen to access the “info” button.
Edit, have reimported and checked in beta now that HA is live & can state that Grazul is still not in the HA pool.
I guess they get added at the end of their initial month (i.e. Grazul was Sept 2019 featured hero and will be available from October 1st 2020).