Only KDE software, I believe.
it definitely needs to be communicated better. See ⚓ T13500 Don't let users install outdated non-KDE apps in Neon. My preference would be for apt itself to either prevent it, or at least warn you in huge red text that you’re entering “there be dragons” territory.
I can also submit an MR to put this info on the website. Once my other ones are merged, that is, to avoid conflicts and excessive drama. 
But really, this shouldn’t have to happen in the first place. It’s only needed because the base OS becomes fragile as a result of all the libraries that have to be upgraded to support newer KDE packages that are overlayed on top. It’s not actually an LTS OS base anymore. If the base were something more recent, like Arch or Fedora, it would all be a non-issue.