My bad, I meant to reply to that and it’s slipped away from me.
Currently they are hard-deleted[1] after the retention period. 
For individual chat message or chat channel deletions, they are initially soft-deleted[2], and we’re adding in the bit that will make them hard-deleted along with the others once the retention period rolls round.
(Small note: Even though the channels are soft-deleted initially, there is currently no way to resurrect them once gone. It’s on the roadmap, but hasn’t been scheduled as yet)