Not spin, it's a fact Gmail uses non standard protocols, even with IMAP.
The spin I referred to is the assertion this is purely a google problem. I did not address, at all, whether Gmail uses non-standard protocols.
To recap: the only thing that changed between completely working and completely failing is the Apple update. Nothing on google's end changed. No client configuration changed. This is not a transient issue since restoring to 10.14.3 works.
The only way your response to my post makes sense is if Apple announced, in advance, they were stopping support for some aspect of gmail because it is non-standard. That would be on google for not making the change Apple demanded and people would have time to switch to another mail client, mail service or simply not update. This isn't the case.