I entirely missed this, and have no understanding of the rationale, but:
1) I'm somewhat ambivalent about removing <received/>, but it's certainly well implemented by my anecdotological studies.
2) Removing <acknowledged/> seems OK, but I've considered various use-cases for which an explicit, user-driven, acknowledgement is useful. But I've never actually implemented it, so...
3) It's really not. Some clients and intermediaries need to know whether they should expect a marker or not. No need to check this prior to sending a <markable/>, true, but useful to track missing markers afterward.
Where was the discussion of this, and (more broadly) how does one know what the discussion and rationale and consensus is if not on this list?
Dave.