I think "standard" needs a lot more detail, here. To me that says "protocol", and I don't see a clear way a protocol could prove text provenance, as any protocol is ultimately only as good as what one side passes into it. Even if you were to somehow seal and stream keyboard events without invading privacy, that's just raising the bar for fraud -- a frontier model could whip up a python intermediary to simulate human keystroke speeds in 30 minutes or less.
I suppose the very hardest evidence to fake would be a video of the author typing, verified by AI (we'll soon lose the ability to distinguish on our own, if we haven't already).
Perhaps even better would be a keyboard with physical yubikey-esque technology... shit, that one might actually work. If someone has a robot dextrous enough to type, text provenance will be the least of our concerns, anyway!
I took a shot at something like what you're talking about, tho it's less "verification" and more "uncovering" information the creator is freely giving away. Undertone[1]: a local Python indexer plus a Firefox/Chrome extension that tints text by evidenced git provenance.[2] I've been too overwhelmed with the job search to finish it in any meaningful sense, but there's freely licensed scraps there you're/anyone is free to use if you'd like :)
But again, to be clear: it relies on good faith alone, and is intended for websites exclusively. Would never coherently work for non-VC'd prose (read: most prose).
[1]: What does it mean? I'm not sure. But it sounds provocative! It get's the people going!
[2]: https://gitlab.com/doering-ai/apps/undertone