Provide an API for prepopuating data on Webform submission forms

I am very hesitant to support or implement any new APIs. I think this type of API needs to be provided by a contributed module

I have seen your answer Jacob, my first and most important question: Do you have a practical suggestion for how a contrib/custom code could add this functionality? Because based on my proposed change it would be only possible by overriding and subclassing \Drupal\webform\WebformSubmissionForm which I would avoid due to multiple potential maintenance traps in the road ahead...

I am very hesitant to support or implement any new APIs.

The other question is why are you...? This project is still active and growing, DX should become better as well, not just the site builder experience. Actually, with this new API we can potentially serve multiple personas.

Because based on my proposed change it would be only possible by overriding and subclassing \Drupal\webform\WebformSubmissionForm which I would avoid due to multiple potential maintenance traps in the road ahead...

I also checked what calls \Drupal\webform\WebformSubmissionForm::prepopulateData() and it is \Drupal\webform\WebformSubmissionForm::buildForm() so we are not closer with that either... unless we try to add this feature via a form alter which is meh...

Just stumbling over this...
I think such an API would be powerful and belongs into webform itself, not a separate module. @jrockowitz I fully understand your hesitance to add new APIs, that makes total sense, but this one would add quite a few new opportunities to webform that justify the exception.

For any new feature, there needs to be significant interest and test coverage.

Currently the webform modules does not trigger to many event and maybe we could compromise and add new (and simpler) hook like hook_webform_submission_prepopulate().