I think what the professor did was exactly correct, have them experience something new mathematically, but give them the heads up that they weren't (likely) to be solving it, as it's a new pattern they hadn't recognized before.

I wonder how well things are likely to work if you give students explicit license to not try in an effort to keep them from feeling failure.

Not knowing is the important part. If you know the question is unsolved, you already have an 'answer' for it: it's 'unsolved.'