I wonder that if I customize fields (for example "due date" field) not "optional or required" but system warns user as "you should fill that field" but users can skip that warning and left field blank or fill that field according to their decision. Is it possible for JIRA to do anything similar to that solution?
I don't think I understand the question here.
The point of setting a field as mandatory is that the users have to fill it in. To let them choose whether to fill it in or not, you simply make it optional. There's no point warning them if a field is optional, because it's well, optional, so they don't need to fill it in.
Could you explain what I've misunderstood in your question? What do you need? Is it more like you want fields to be optional when the issue is created, and mandatory later?
Oh, those things in SAP, yes, I've seen them. They're horrid in UI terms - they slow the user down with information they don't care about (broadly users fall into two camps - "don't understand the warning" and "don't need the warning") and add work that they don't want to do having to click or confirm. In cases where the field is really important, you'd want it mandatory anyway and handle that in validation.
You could perhaps build your warnings into the field descriptions (via Field Configurations).
Separately, there is an open improvement issue JRA-5783 (Make field required only for one state transition) which will be great for making indivdual fields sometimes required and sometimes not required, all depending on the transition. That is not quite what you want unless "left field blank or fill that field according to their decision" includes letting the system make the decision.
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