There was some Friday afternoon goings on on possession claims after all. There are new accelerated possession claim forms on Gov.uk, added on 11 September 2020.
The N5B has been missing since 17 August! I don’t know why this has taken a month to resolve, (and people will have been using the old form in the meantime), but it is here now.
The previous N11B was wrong in all sorts of ways, so this is a welcome correction.