> Unfortunately Unicode retconned the ASCII apostrophe character ' to be a vertical line

Unicode does not precribe the appearance of characters. Although in the code chart¹ it says »neutral (vertical) glyph with mixed usage« (next to »apostrophe-quote« and »single quote«), font vendors have to deal with this mixed usage. And with Unicode the correct quotation marks have their own code points, making it unnecessary to design fonts where the ASCII apostrophe takes their form, but rendering all other uses pretty ugly.

I would regard using ` and ' as paired quotation marks as a hack from times when typographic expression was simply not possible with the character sets of the day.

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¹

    0027 ' APOSTROPHE
    = apostrophe-quote (1.0)
    = single quote
    = APL quote
    • neutral (vertical) glyph with mixed usage
    • 2019 ’ is preferred for apostrophe
    • preferred characters in English for paired quotation marks are 2018 ‘ & 2019 ’
    • 05F3 ׳ is preferred for geresh when writing Hebrew
    → 02B9 ʹ modifier letter prime
    → 02BC ʼ modifier letter apostrophe
    → 02C8 ˈ modifier letter vertical line
    → 0301 $́ combining acute accent
    → 030D $̍ combining vertical line above
    → 05F3 ׳ hebrew punctuation geresh
    → 2018 ‘ left single quotation mark
    → 2019 ’ right single quotation mark
    → 2032 ′ prime
    → A78C ꞌ latin small letter saltillo«