Thanks to a tip by Jamie, you can use eBay's Mobile Friendly Test with any of your item numbers to see if they're up to snuff for people shopping with Androids, iPhones, Tablets etc. None of our listings were passing the test, despite having recently transformed them all into plain text with no active content or html. The great thing is, it's easy to bulk edit their code right into your listings, just follow the tutorial above posted by fellow eBayer Mesha Travesser.
The most obvious question is why aren't listings generated on the eBay website that just use plain text passing this mobile friendly test? The asnwer is: I have no idea. This is clearly a run-up to when eBay discontinues the ability to put active content into your descriptions. They want every description to be pure, plain text for mobile users. Hopefully, next year this code will automatically be part of their internal code for all listings. But for now, it won't take long to bulk edit, so you might as well give it a try!