That's not correct. MHTML has been defined as internet standard since 1999. MHTML has been an internet standard since 1999, with support dating back to IE 5.0.
[1] https://docs.google.com/document/d/1FvmYUC0S0BkdkR7wZsg0hLdK...
Are you meaning that these modifications cause the generated MHTML files to become non-standards-compliant?
I wouldn't consider them proprietary though. At least the primary software used to open these files, Chromium, is open-source software.