Currently, there are no system restrictions on when "admin" and "local" can be dropped. Because the usual protections against writing/dropping system collections are skipped (e.g. dropping a "system.users" collection is normally prohibited), it is too easy to break a running system with a simple dropDatabase command.
One way this can manifest is by dropping the admin database while a "system.version" exists in it, which can downgrade your feature compatibility version.
To work around this new restriction and drop these databases, one can restart a node in standalone mode.