Solved!
I previously had created 2 working rules in Outlook 2013, both of which deleted incoming emails based on specific words in the subject; after upgrading to Outlook 2016, 1 rule quit working.
I compared the previously-working rule to the rule that still worked.
Under Select condition(s), both had check-marks by 'with specific words in the subject'; only the words were different.
Under Select action(s), 'delete it' was checked in both rules, however, the non-working rule also had 'mark it as read' checked; I assumed the latter had nothing to do with why this rule no-longer worked.
After searching the web and trying everything including deleting and re-creating the non-working rule umpteen times, my 1st 'solution' was to add the non-working rule's subject-area words to the working rule's list of subject-area words; which worked immediately.
A few days ago whilst perusing the Deleted Items folder prior to emptying it, I noticed a ton of emails with bold headers - I.E. marked unread - that had been deleted by the working rule, and so I edited the working rule, adding a check-mark under Select action(s) beside 'mark it as read'.
When I fired-up Outlook 2016 the following morning, the (previously) still-working rule no longer worked (although it successfully worked when I manually ran it).
Though I couldn't believe the previous day's change was the culprit behind the rule's failure, I nevertheless un-checked 'mark it as read' in the Rules Wizard under Select action(s), then I saved my change and reopened Outlook.
The rule once again worked and continues to run every day as expected. After un-selecting 'mark it as read' on the other rule which had stopped working, it too ran again without failure. I don't know why deselecting the 'mark it as read' action solved my problem, I'm just glad it's fixed.