“Supposed to”? That originated back before desktop publishing existed, when typed communication (as in typewriters) were only monospace, and adding 2 spaces between sentences made text easier to read.

In modern times (as in the past 30 years or so, i think), while adding 2 spaces between sentences might remain as a preference or in a style guide, you haven’t been “supposed to” do it.

Maybe now that markdown is so commonplace, it’ll make a return?