Did you try it with kdesrc-build's Qt build? For some reason it doesn't work for me with Qt::X11Extras.
I'll file an issue if it's not my local problem.

I did not test it, but I don't see why it would behave differently, assuming it builds Qt 5.15

after this mr landed, kwin was stilling building fine in CI. it must be a local setup issue.

Does our CI use kdesrc-build's own Qt build and not distro-provided version?

Just got it again, on different system:

-- Configuring done
CMake Error at src/CMakeLists.txt:279 (add_executable):
  Target "kwin_x11" links to target "Qt::X11Extras" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?
...

As workaround I had to revert Qt::X11Extras to Qt5::X11Extras everywhere. Could someone try to reproduce as it still might be non-local issue maybe?

Could someone try to reproduce as it still might be non-local issue maybe?

We are all building kwin just fine. There's been some issues in the CI, but due to a different issue.