Since I updated to 12.3.1, those artifacts - with which I've lived since
January 2017 - have disappeared, and since Office 2015 hasn't been supported for a year or more, whatever has changed must be Apple's doing.
Conundrum: Since I've never had the slightest idea what, if any, purpose the artifacts served, I don't know if I've benefitted or lost functionality.
At any rate, though, I never realized how visually offensive the artifacts were until they were gone, and now... Wow! What a difference.
As a matter of fact, the difference is so stunning that I'm amazed that it's taken me the 10 days since I ran the updater to notice it. I give up! ¯\_(ツ)_/¯
(Note: I've never paid attention to how long a period of time is documented in /Library/Receipts/InstallHistory.plist, but I just noticed that it was wiped when I updated to 12.3.1. That's a shame, and I hope it's only a temporary glitch, because despite how infrequently I need it, it was a useful database to have available.)