Remove the Gmail service from your Google Account. In order to be able to do this you will need an email address that is not used as the primary or alternative email address of another Google Account. For step by step instructions please read the official help article:
Delete your Gmail service
Can I re-create deleted Gmail accounts?
Need to ...
Regarding the "Can't reach this page", the most straight forward solution is to clear the cookies and delete the cache of your web browser. Then sign in again to your account.
Regarding being lock out from time to time, for security reasons, Google will ask to sign-in again in the web browser every certain time.
"Perhaps the ability to sign out of a single account has been removed?"
I am afraid you are correct.
It has nothing to do with the kind of the operating system (Mac, Windows, Linux etc) you use.
It also has nothing to do with the browser you use (being Chrome, FireFox, Safari etc).
It is ALL about the browser you are logged into from.
As per the ...