I didn't realize my "business typing" class, which I took ca. 1968, was all
that close to the Industrial Revolution. ;-) We were taught to space twice
after sentence-ending punctuation. This was to improve readability when
using fixed pitched fonts.
It wasn't until 1988, when I began writing for publishers that I was told
that two spaces were verboten when using proportionally spaced fonts. Hence,
all of my publishing clients since that time have insisted on single spaces
between sentences.
Even so, many who were taught to type in the 1960s and earlier, but who
never wrote professionally, were never taught the single-spacing rule when
using proportional spacing. 40+ years of habit are hard to overcome, and
when they moved to word processors, they simply kept on doing what they'd
already done.
You *can* tell Word's grammar to mark 1 or 2 spaces as a "grammar" error, if
you like. Tools - Options - Spelling & Grammar - Settings. But, getting
others to use a single space is awfully hard to do. I've given up trying to
convert others, and simply routinely replace all double spaces with single
spaces when I receive document from typewriter-trained colleagues.