I hadn't seen that before, but figures. More and more, a nice Lenovo, Dell, or System76 laptop running Linux is starting to sound really good. Hell, it's got better gaming support than Mac now, with Vulkan.
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.