ANSI: the American National Standards Institute, a private non-profit group that oversees the creation of voluntary safety and product guidelines in the United States. Including ASCII. Pretty sure they own plain text, in the sense that OP uses it.

And then ISO, which owns a bunch of extended ASCII encodings that provide essential support for "plain text" in languages other than English (and an ugly historical hostility toward non-European languages).

And then Unicode Consortium, which owns Unicode, its ever-expanding list of complexities and exceptions and an ugly history of hostility toward Asian languages.