Jon Peterson's Playing at the World blog has a post called Dragons at a Distance: Early Play-by-Mail D&D, which may seem especially relevant these days. One question I wonder about is whether anyone was running a mix play-by-mail/play by phone campaign back in the day. Regular long-distance phone calls would have been too expensive back then, but you could imagine someone handling exploration and negotiation through the mail, switching to phone when a combat breaks out.
These days, people do Discord/Skype/Hangouts/Zoom, but that generally requires scheduling a session. One could imagine handling a larger number of players via email, with players teaming up as their schedules permit for actual expeditions.