If you mean the red underlines on misspelled words, it's been like that for at least as long as I have been posting. I always use the "quick reply" box at the bottom, so I'm not sure how spell check works if you post a different way.
I don't think the forum does underlining or autocorrect? (although I block most scripts so it could be one I'm missing.) If you are using Firefox it has built in autocorrect and underlining that you might be seeing - and there was a new version out a few days ago that might have turned it on for you?