Well, looks like the Monotype Library Subscription app has been abandoned. I can't find anything about updates to it online, or indeed any support page for it period. Thankfully, I can still access and use fonts via SkyFonts, which I already have installed. That app, at least, is still supported. Still a pain, because now I have to uninstall the fonts I installed using the MLS app and reinstall them via SkyFonts.
Truth be told, I won't miss the MLS app, as it was temperament at the best of times.