Thanks for getting in touch! We do not currently delete/deactivate users based on Active Directory status.
Until the Subscriptions functionality was introduced there was little benefit to be gained by constantly polling AD to see if users had been disabled/removed. Once the user was disabled/removed in AD they could no longer log in to the network and therefore couldn’t access Octopus, so there seemed little harm in the User records remaining unused in the DB.
However, now that Subscriptions has been released, there may be a need for this functionality to stop emails going to users who are disabled/removed. We don’t currently have plans to add this functionality (it’s not trivial, to answer your question), but we’re always willing to take input from the community on prioritizing features so I’ve created an item in our UserVoice. Please vote for it if you feel it’s important.