And this support-and-opinion asking ticket follows the 3rd point of WHATWG process description ( https://wiki.whatwg.org/wiki/FAQ#Is_there_a_process_for_adding_new_features_to_a_specification.3F ): "Get more people involved. Ask fellow Web developers about their opinions (but remind them of step 1 above [= don't think about how this should be implemented])."
I agree with what @domenic said in the github issue. We already have a bug open to do implement a warning for password fields: bug 259059. I guess we could warn for ordinary text fields as well on focus, and then remove the warning once the user starts typing (because then it should obvious).
In fact, I agree too because it would not be as powerful as a new attribute, but it would be way sufficient for the vast majority of use cases (passwords are usually case sensitive, but usernames are usually not, but they're right: I'm now unsure about how UX would be improved by knowing the username is not case sensitive).
According to the conclusion https://github.com/whatwg/html/issues/2064 going to close this as wontfix, but let's move the UI discussion to bug 259059. Thanks for the discussion!