I just stumbled upon two releases from the time classicalarchives.com was selling tracks individually, and grouped them only by work, not by release (Archive.org). The website has changed, and it seems the two releases come from this album (Prestoclassical, russiancd). I’m wondering if we should keep this kind of release, or merge them with the complete album. Does this qualifies as a release under the current guidelines ?
In other words, should we have a separate release each time a retailer (arbitrarily) groups tracks differently ? Looking at the old classicalarchives.com page, it’s not obvious to me that each work is a release especially since complete works are available for streaming, not downloading, but I can see arguments for keeping such releases too.
Once something is released, it’s a release. You can’t unrelease something. 
As unwieldy as Classical Archives’ old policy was, it’s a fact that should be reflected. I wouldn’t advise anyone to add all these old releases though. These are releases with a very limited distribution.
So I would keep the old releases as they are in MusicBrainz now, add the new “full” releases if they aren’t there already, and merge the recordings.
A nice annotation noting why they’re grouped this way, where they came from, and that they might not be available in that way anymore, is super useful as well, especially if consistently applied to all releases from that site! 
I suppose my question was more “Was this a valid release to begin with?” than “should we delete it because they don’t sell it anymore?”. And I guess the answer is yes, thanks for the help.