PINE (the mail client) had a ^J keyboard shortcut to justify the lines. It's been a long time since I used it, but I seem to recall that it would insert double space after period when you hit that key. It might even have been possible to set it up to auto justify on save/send. I suspect that's why a lot of old (plain text) email had the double space after period and perhaps still does in OSS circles.
I think the default vim justification worked that way too. Around this time I went through a phase of bloody-mindedly using Mail/mailx and vi on OpenBSD while still sending mail through my ISP's SMTP and using fetchmail to grab it through POP3. I would not at all be surprised if cypherpunk types were doing the same thing, even if their main desktop or laptop was Windows and they used single space after period for non-email communication.
Edit to add: I just looked up fmt(1) manpage[0] and it specifically mentions using it to format mails, and that the default is two space after period.