I just tested stell ceck on my computer (latest Windows 11 [25h2]) and Firefox. The misspelled words in the previous sentence have squiggly red lines and give suggestions when clicked.
Standard that came with Windows 11 'Microsoft Edge'. It used to warn me if words were misspelled. Not now. I tries some window's fixes but nothing has worked.
Edit: I have just tried other site and such. No spell check in them either. Gulp.