Is it okay for questions to have just plain sudokus? Or do they have to have a element in them?
Because, I don't see any posts with just plain sudokus, but are they allowed?
As long as the puzzle is either
there are no rules against posting just a plain sudoku. (In fact, here's an example.)
That being said, it would likely not be well-received. Users of this site aren't here looking for sudokus; there are many sites out there that provide as many sudokus as anyone could want. This site is more about the creation and solving of puzzles.
Thus people have posted questions asking
So the TLDR is that it's not off-topic, but probably won't be well-received.