Since forever. It's very slow, I'm still not sure replying is actually in the spec or email clients fake it with Re: and then force you to quote the entire history of the conversation back and forth. Also very easy to break if you don't like the Re: or something. People are constantly replying to the wrong person or persons, sometimes even to themselves. You have weirdly named fields "cc" and "bcc" that are present all the even though I use them like 4x and 1x a year, respectively. You can't unsend or delete emails.

And all this is before I get into doing git or calendars over email.

Email is in fact one of the reasons I'm not sure I want the fediverse to succeed right now, because then all the faults of activitypub will be forced on us for centuries, like they are with email.