This didn't work for those of us who map Caps Lock to Control, like god intended. IIRC, it wasn't possible to map the left Control key to ESC for some reason.
1) install karabiner elements
2) select complex modifications
3) add rule
4) import more from internet
5) import change caps lock key v4
6) add rule change caps_lock to control if pressed with other keys, to escape if pressed alone
...is my actual current setup, along with a bunch of other key mapping. But I started with what I posted above, which everyone with Mojave+ on a MBP has available already.
I'm very afraid of getting too used to it and then having it stop working.
As far as the immediate concern goes, Karabiner is such a popular thing that I suspect there'll be some way to support it. If not, I won't upgrade. I just bought this MacBook Pro, and it'll probably continue to be usable on 10.15.x for its lifetime. I'm not too interested in my gaming keyboard and 10 key pad becoming unusable, at any rate.
If they actually completely kill software support for this, I guess next best thing would be a USB intermediary to process the key codes. Possible it's doable with a Raspberry PI or something, and there are enough mechanical keyboard enthusiasts on Mac someone would be motivated to do it. Won't help for the internal keyboard though.
It’s a “simple modification” (specific type) in Karabiner though, if you use that. I use PC mechanical keyboards so most of my typing devices a) have swapped around mod keys and b) map right ctrl to fn so I can do Mac device hotkeys from pc keyboard.