Thanks for the suggestions Chris, if I was just after downloading the installer and getting on with my life, the release archive would be the way to go.
However I’m more concerned with trying to get someone responsible for the repo and documentation to make some changes. In the interest of saving others time in the future.
I appreciate I could just pull the deb package from the release archive, and if Mongo are intending on that being the way forward for old version (3.6 IS old) then so be it.
However If the intention is to cease supporting the Ubuntu repo, then surely to avoid confusion the best approach is to amend the community docs and remove the repo from the internet.
If the intention is to keep it around then they key should be updated.
The current situation is a bit of a mess and I can see it wasting peoples time when they come to try and install from the repo and get the signing key error.
I gather as a lowly non-paying punter the best I can hope for is someone from Mongo reading this post and maybe takign some action. So fingers crossed.
P.S.
I’m not sure of the wisdom of marking a public repository as trusted in the absence of a valid signing yet as suggested in some of those linked topics!