I'm curious as to hearing experiences with 17 and earlier on El Capitan as well.
I've put off upgrading to El Capitan mostly because I have been too busy to make a clone of my Yosemite system to guarantee access to older versions of ArchiCAD, the only software I have that seems to have issue with OS upgrades.
Graeme, I strongly encourage you to get an external disk and clone (and then test the clone) your Yosemite system to it. (You can also clone to a partition of an internal disk.) This will permit you to reboot your Mac with Yosemite if you encounter issues with some version of AC.
2014 review of various clone apps:
http://www.macworld.com/article/2461362/drive-cloning-utilities-the-best-mac-apps-for-making-a-boota...
[I've used all of the methods listed, and pretty much agree with the reviewer's pros/cons.]
Booting from a clone is easy but switching back and forth would get confusing, so I encourage this mainly for people like me who MIGHT have to run an old version rather than folks who EXPECT to have to run old versions somewhat regularly. Once you have clones, here's how you boot from one:
https://support.apple.com/en-us/HT204417
Be sure to avoid 'clone wars' in Spotlight and Time Machine by setting each to ignore the (other) clone volumes. Otherwise, you'll see multiple copies of each app/file in Spotlight/Finder Find and won't quickly know which one is part of the system you've booted to.
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AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB