Reddit saves your keystrokes in text boxes

They provide automatic saving of comments drafts in case you return. Is it possibly abused by analysis, sure. Do they let you know that they're saving drafts, pretty clearly.

The standard html behavior of only an alert does not handle much of. the user desires.

Reddit captures everything you are doing including your whole hardware and attempt to fingerprint you hard, so yes, keystrokes are included.

I would never trust a webpage to not send out all possible information, why wouldn't they do it?

I noticed this awhile back. Check out the network traffic during typing. https://postimg.cc/k2tbxQSx

Is it a RUM analytics/debugging tool like those from Datadog and Sentry (potentially behind a proxy)?