Hi,
So what issues are you having with Mail ?
Much earlier than the date mentioned in the Email Google decided not to use SSL any more.
At the time SSL (Secure Socket Layers) was considered to be one of the more secure ways to login in to web sites and severs.
However there was one of those episodes where someone found a flaw that could be exploited.
Most organisations fixed this and moved on but Google started using the OAuth token that the email talks about.
They also allowed people to still use SSL by a setting on your Google account pages for Security and setting "Allow Less Secure App"
Web page logins to the Google Mail server still uses SSL.
The Mail app has also done it for long time irrespective if you set "Allow Less Secure apps" or not.
However the Messages App using a Google ID login has always used SSL and nothing else and has needed "Less Secure apps".
For this reason it is not clear on my system if the Mail app to Google's mail server is using SSL or not.
On the basis that every other Jabber/XMPP server for Messaging using the Jabber protocol uses SSL to login (only Google are different) then I cannot see Apple changing this.
AS that would still only be one app if they did I can't see them changing Mail to use anything other than SSL in Mail (It works with every other server).
That would suggest you will have to enable Less Secure Apps on your Google settings.
Not that I have been looking but I have not seen anything that does state that Google's OAuth certificates are any more secure than the latest SSL ones.
8:25 pm Wednesday; August 10, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)