Why do you think this is spell check and not syntax checking?
The tab shows as unsaved up at the top and there is a JS next to that, which probably means it's trying to parse that as JavaScript. Just save the file with an appropriate extension (maybe .txt or .md) or you can (I think) also manually set that behavior.
Yeah this is why. What you can also do is click the language in the status bar at the bottom right to switch language without having to save it to a file.
I don't think it's spell check. For whatever reason, VSCode decided that you're writing JS code and is trying to point out syntax errors. There should be some way to change the file type (open the command palette and look for "file type", I suppose)
The 'untitled' heading makes me think that's an unsaved file - I think it only has spell check in non-syntaxed unsaved files. It's been years since I used it but I never had it yelling at me in regular code 🤔
Ew. Is that a text editor? (I use KDE and that kind of UI WTF would irritate the hell out of me enough to switch, too! I don't use kate, though, if that's what that is.)