Our Chrome web browsers happened to update overnight to version 142.0.7444.60. Now, whenever we try to access an WAB using the Classic Web Map Viewer (or any map using the Classic Web Map Viewer), none of our hosted layers are available. Everything is visible and functional still if you open the map in the new Map Viewer, but anything build with Classic seems to be broken atm.
We still haven't finished converting all our WAB applications over to Experience Builder yet, as I had assumed they would continue to work until next spring.