That’s just a bad syntax choice on Gruber’s part. CommonMark adds trailing backslash as an alternative, so that will work in most places these days.
> And even this is renderer dependent, I don't think is part of the spec (?)
Yes it is. Quoting https://daringfireball.net/projects/markdown/syntax: “When you do want to insert a <br /> break tag using Markdown, you end a line with two or more spaces, then type return.”
> IIRC GitHub comments renderer does't need it
Yes, GitHub decided on a wilful violation of Markdown for issues and discussions.
> text editors which auto-convert double space into ". "
I have seen that as a feature on Android keyboards, but I would be very much surprised to find it in non-keyboard software.
I believe Gruber was inspired by how people wrote emphasis in plain text emails and other text documents. Most MUA at the time would treat trailing whitespace as a hard rather than soft line break. This is from my--now aging--memory, and I can't find a source to corroborate. I do recall, though, there were clients that didn't do it well (ahem, Outlook), and would break plain text formatting of deeply-nested quoted text. (Don't even get me started on how Outlook single-handedly changed culture from bottom posting to top posting).
Or how it single-handedly kept HTML for email frozen with an incomplete and buggy implementation of HTML 3.2 from 1997…