I strongly disagree with this. We have an API for this stuff https://docs.joomla.org/Server-side_form_validation with JFormFieldRule - this PR is encouraging bad practice. At validation of data you should NEVER be modifying the form definition OR the data itself - it should be done in the existing plugin event onContentPrepareForm.

This hasn't to do with validation it allows manipulation and this is need when you try to implement something like this #10347

I have seen that plugin and in there you are not using onUserAfterDataValidation and as far as I can tell the code in onUserBeforeDataValidation could be moved into onContentPrepareForm without affecting anything

We can remove the onUserAfterDataValidation I only though it might be good to have a change to get the validation result and maybe to access some of the error messages. For the other event you can move it but then it will not work because the validation works on the requestdata see https://github.com/joomla/joomla-cms/blob/staging/components/com_users/controllers/registration.php#L137

and you can't manipulate this in onContentPrepareForm, here you get plain nothing beside the default group mapping

@pe7er Peter if you have a chance can you take a look here if this addition would allow you to write the plugin you tried to write, thanks.

@rdeutz Ok, I will do so when I can make some time (which is not before JAB16)

See #10751