You can add a virtual column to the table containing your dates with an expression similar to what you want to use in your formatting rule. Something like:
IF([Order Status] = “Complete”, “Green”, “Red”)
I can’t speak as to why you can’t use a formatting rule for this, but I can say that I use the calendar implementation with extensive color formatting using an extra column, and it works well for me.
“Abandoned” is perhaps too strong a word. “Deprioritized” would probably be better. Still, it’s reasonable to expect neither onboarding nor calendar views will receive too much developer attention in the foreseeable future.