Basically my worst fears while working on the modifier changes have realized themselves.
I inadvertently removed a property from a modifier and it wasn't caught in time for the release!

This post on devtalk brought it to my attention: https://devtalk.blender.org/t/decimate-modifier-and-other-things-short-feedback/15162

I'm not sure quite what the procedure is for getting this in 2.90.1, but I added it to the list in T80396.