After this post (and some substantial behind the scenes contacts from others), MHCLG have acknowledged that the amended Form 6A that had been put up on the gov.uk site should indeed not have been put up. The form 6A has been replaced with the version in the May 2019 The Assured Tenancies and Agricultural Occupancies (Forms) (England) (Amendment) Regulations 2019 (or rather that version has been re-instated). The form 6A on gov.uk is again safe to use. There is a note on the page, as follows (which is slightly confusing, as the 8 August ‘update’ was the amended form, which it now appears was apparently uploaded silently on 30 July.)

So there we are. Both the ‘How to Rent guide and Form 6A snafus have been corrected. It really shouldn’t be down to eagle eyed watchers like Rich Greenhill and David Smith to have to spot these things, though.