Header text is populated automatically because of some enhancement
I don't think so. System, with standard configuration, will populate header text automatically. In your case, if it is not happening, then in that case, some exit should be in place. Check your system.
How to make Header text non-editable (only display) in VF02?
Do you mean to say, currently, it is not editable? You should be aware, if an accounting document has been generated, in that case only, the header text in billing document would NOT be in editable mode. How about in your case?
G. Lakshmipathi