As someone who works in infrastructure I disagree with this. While it’s true that applications should always have done this, many many do not and need to be dealt with. If they are third party paying customers, your only option is to figure out how to make things work, because asking customers to upgrade will piss many of them off.

The simple fact so many applications do not treat headers as case insensitive is one of the major things holding back http 2. With HTTP/1 it was suggested but not enforced. Upgrading to http/2 with forced-lowecasing breaks these applications