I belive Knuth wrote Tex of which LaTex is an extension of.
Ah you are correct.
A proprietary file format is one where you have to have software that can decode the information from a binary file. Don't know the encoded scheme or have no software that does and you can't access that document.
Plain text may be the only truly industry wide document standard. Underappreciated in my opinion. Also one of the reasons I like using *nix. The well documented plain text configuration files in the /etc folder is so much more sane than MS Windows registry.
You obviously have a really screwed up view of what a properietary format is. All files (even text files) require software needed to decode information in the file.
A text file is meaningless without a decoder which converts the bits into human readable symbols. The fact that there is a standard is what makes it non-proprietary.
It doesnt matter what the specific mapping of bytes to symbols is as long as we all agree. If Joe Minority used a different mapping then the rest of us, then Joe Minority's format would be proprietary.
Under your definition:
All graphics formats are proprietary
All audio formats are proprietary
All document formats are proprietary
and infact:
Under your definition all files formats, now and forever in the future, are proprietary.
This sort of thing extends well beyond computers. Standards and standards riding upon standards is the foundation of a civilizations infrastructure.
In America we have the 110 volt (a standard unit of measurement) electricity standard, which runs to a standard wall outlet, a standard plug connects to the outlet to provide power to a myriad of electronics. Inside the electronics are standardized modules placed on PCB's. The electronic device is made from a standard plastic, or coated with a standard paint, and the whole thing conforms to a standard electromagnetic noise limitation.
Imagine a society where there was no standard wall outlet or no standard voltage.
I re-iterate my previous point:
It is the fact that there are competing standards that creates the frustration in regards to document formats, audio formats, archive formats, graphics formats, and so forth.