Computer text tradition aside, nobody seems to offer any legitimate reason why such information isn't worthy of being preservable in plain-text. Perhaps there isn't one.
By introducing state, even localized, you are creating a de-fact0 "rich-text" protocol - unless you duplicate all code points in italics, what you create is 'not so plain' text.
VS's and similar efforts are pseud0-encoding are already in a gray zone, but at least for VSs there's an established protocol how to ignore presentation issues in processing.
Much of the discussion of 'plain text' here is very focused on presentation and does not adequately consider the harm done to the text-processing model that underlies Unicode.
That's all I'm prepared to contribute for a bit.
A./
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