Is there a function to keep draft before submitting a question? If there is not, could you consider to implement it?
As long as you don't delete it, SE seems to do some sort of autosaving automatically. I had a unposted, nonsense post follow me across systems. If you want to ask multiple questions as drafts, you have an issue, but as long as you're logged in, your partially asked question will follow you around.
Is there a function to keep draft before submitting a question?
Not yet, I am afraid. However as long as you do not delete it, there would be a temporary memory in our browser. One of the alternative solution is to use editor, eg. https://stackedit.io/
If there is not, could you consider to implement it?
I hope the Stack overflow developer can consider this brilliant idea. I should think more to post the question when it is not ready. Presumably it will leads to down vote.