Each channe should have post threshold levels …
Keep a maximum of NN posts
Keep posts for a maximum age of DD days
Keep posts for a maximum age of DD days but no less than NN posts
If nothing is set, a default (global) set of (same) configurable values should be used.
Data retention (and privacy/encryption/need-to-know-only-basis) will be a big issue come May 2018 when GDPR hits all of EU. But the automated channel maintenance is something MM needs regardless of GDPR.
As an option, it’d be nice if there was a selector between purging/pruning and archiving and then allow for a “Keep archived blobs for TT time”. An archive blob could be moved to secondary or offline storage, in which case it could be marked as such and then re-imported manually by admins if needed. Encrypted, yes of course