I seem to have gotten stuck in MacOS / Xcode purgatory.
I'm on OS Sierra/10.12.6.
When I try to update, I get a warning, that there's an issue with upgrading vs DisplayLink and so it cannot be done.
That directly conflicts with Xcode which (as of now, on App Store) only works on 10.13.2 or later.
I cannot find a place to download an earlier version of Xcode. I cannot upgrade to High Sierra.
How can I get this happening?