At the time of my initial posting, I think I inadvertently was subscribed to email notification for ALL new forum postings rather than replies to my particular topic. Since then, I have been receiving a high volume of forum email notifications of no interest.
I do not see a button anywhere that enables unsubscribing, either per category or 'unsubscribe all'. I only see the unsubscribe link for each topic.
Guess what, after posting this, I see an inconspicuous Stop following forum link at the bottom of the page. I clicked and will see what happens.
This is a problem with the user interface, we need to make this work better. We have an open issue on GitHub and discuss the proper way to unsubscribe here:
Github Issue: https://github.com/backdrop-ops/forum.backdropcms.org/issues/55