Kate used to highlight misspelled words when automatic spell checking was enabled. This broke on my system. I don't see anything visually different about words with spelling mistakes, and the usual red line is missing.

The underlying spell checking seems to work: the Tools->Spelling->Spelling... menu entry will find and ask about bad words. They're just not rendered with the red under line (apparently called "spelling mistake line" in the syntax highlight setting).

This is a rather old issue, I only just reported it now because it was something so obvious that surely would be fixed soon. Plus Debian remained on an old kate version for ages. But it also happens with the official appimage and when I built a kate release from git (a slightly older release because git master required some weird newer KF stuff).

Surely this is some sort of configuration of packaging issue, but I'm out of ideas.