What I provided above and what I've captured here:

are all that I can see under File/Office Account/About Outlook.
I'm sorry I don't understand anything about channels in the context of MS support. I just buy and install products and hope for the best!
Despite my frustration I do appreciate your ongoing involvement and determination to help. I will really be glad when I get my Horizontal Line back again as it worked for many years dating back to some earlier Office 20xx.
By the way, Horizontal Line works fine in my copy of Word 2016.
Also, while Horizontal Rule and Bottom Border look quite similar, they appear to be quite different beasts, in the sense that Horizontal Line is selectable and deletable in its own right (like a piece of text) whereas Bottom Border can only be turned on
and off but can't actually be selected or otherwise manipulated.