> The Unicode Standard defines plain text essentially as a sequence of character codes, without the additional formatting information associated with rich text. Fonts, colors, layout, and similar presentation details belong somewhere else.

Nope. Skintones are part of Unicode. It also has 33 control characters from ASCII including one that rings a bell... There are also numerous characters added by Unicode that are literally called "layout controls".

If you want a format that provides purely semantic information, then "plaintext" doesn't fit the bill.