Yea, main and off-hands, will result in an error showing on the plate application if not used on the same class. It did actually do it pre-6.1 (I have a glam that i use for multiple healers, but has a specific whm weapon on it that I would receive the message before 6.1 as well). I do think that the warning UI is bigger than it used to be though, and maybe a brighter red, because it definitely pops more than it used to.

*edit* I think the message was a much shorter message as well. but don't recall the precise wording