Yep, I use vimwiki where it's all monospace, and two spaces between sentences is a massive readability benefit there. I also type it when making comments here because these input boxes are also monospace, even though the extra space isn't displayed after posting.

No, the two-space rule was used in books a ~century ago. That's what I'm comparing with.

The habit was adopted for monospace typewriters precisely to try to match book typesetting.

Then the books dropped it, but it persisted in typing habits.

Then once Mac/Windows allowed anyone to use proportional fonts, we were taught to use a single space again.