I think this also gets muddier and muddier because A. LLMs do get better at writing like a human, though slowly (some analysis posted here lately showed pretty clear distinctions in pacing, word-choice etc; not bothering to look it up right now) and B. human language is fluent and people start adopting AI speech!

There are so many levels to this: fully automated AI reply, human sending AI-written reply, human sending AI-written but hand-improved reply, human sending hand-written but AI-improved (or translated) reply, human who reads AI output all day hand-written reply, human who doesn't interact with AI (often) hand-written reply.

Some of these will give you an uncanny feeling, but you always keep guessing who you're actually talking to. In the internet nobody knows you're a dog.