Such a function might be useful, but not very much in my opinion. What would I do with the information: "the domain of this login entry is no longer valid"? It would make me verify this entry, investing time. On the other hand, what will happen if I ignore and don't verify such entries? Nothing! And I save some time. It doesn't change anything - only some more entries in my database that have no use any more. Not much difference between an entry with an invalid domain and an entry I once signed up but never used again and never will. I don't care about this, since 1Password has no limit of login entries.
From time to time, I actually check my database and try to remove stale entries. However, the validity of the domain isn't relevant for that check. I sort all entries by change date, then visit the websites one by one and try to login. If it still works, and I assume I still have some use for that website, the entry remains. If I cannot access and login any more, I delete it. If the domain is invalid, the browser will show me.
For this check, I sort all entries by change date (unfortunately, the last use date is invalid, because it isn't cloud synced) and work from the oldest to the newer entries and check their validity.
An automated domain check could also result in invalid results, because it relies on DNS queries, and if there is a network issue during such test, a still valid domain might be reported as invalid.
If I would be given the opportunity to decide if the developers spend their time on this feature or on some other feature, I would ask them to make the last use date useful by cloud syncing it. Not to add a domain validity check.
