Taking into consideration bugzilla:6614 (from 2006) and Wikipedia talk:Delete unused username after 90 days#Straw poll:
The software should delete old, unused accounts that
- were created more than four years ago from an IP address (not created by another user)
- have no edits, deleted or otherwise and no log actions (other than account creation)
- Could we make a very narrow exception for edits to a now-deleted user page, if that's all they did?
- have never been logged into since creation (if not technically feasible, were last logged into less than a week? month? after being created)
- have no preferences set other than e-mail and password (i.e. what the signup page asks for)
- have no user page or user talk page (doppelgangers)
- Not sure about this one. Certainly, accounts with existing user pages should stay. But it is possible that some users were simply welcomed or served with a block notice on their talk page - in which cases the account is pointless to keep.
I don't see how it makes any difference whether the account is blocked or not. Probably most blocked accounts that fulfil these criteria will be username blocks (since the accounts can't have edited), and it is probably even better to be deleting "crap" usernames (to quote Rob Church).
The main purpose of this would be to free up account names for new users.
It would be nice to do this before the SUL finalization, but I fear that may be happening too soon. (Although SUL finalization will be nice, because a lot of really old, unused accounts will get pushed "out of the way" to xyz~enwiki names.)
It would be helpful to see a list of accounts that satisfy these criteria. An SQL query could be run on toolserver or labs.