Apple wants all programs to run 64 bit-
It already killed Media Pro that is still my favourate local catalogue program.
Programs have to be rewritten to 64 bit or die. I have heard that can be a lot of work so sometimes it is just too expensive.
Aperture is 64 bit so that is not the issue. They are not updating the libraries to support the new OS and eventually things stop.
Unfortunately they have just decided that all photos are now taken on mobile phones.
Apple used to be the most backward compatible OS around.
Apart from Aperture (64 bit but unsupported), I have accounting programmes that are 32 bit and just too expensive and integral to discard.
I have found that you can run High Sierra in a window under High Sierra using VMware. That may be an option going forward.