Why can't I see subscript "s" (Unicode Character “ₛ” (U+209B)) even though I can see lots of other unicode characters? So you can copy paste even if you can't see it - subscript &quot;s&quot; (Unicode Character “ₛ” (U+209B))
My main meta question Should I edit the squares in this title? Is it just me who's seeing them? explains that when I view the site https://www.compart.com/en/unicode/U+209B
Question: I use unicode characters from compart.com and elsewhere frequently (greek letters, arrows, degrees symbol) and I've never had a problem, what is suddenly so challenging about this subscript s?

I see this in MacOS 10.11 and Chrome on a late 2012 MacBook Air

and this in MacOS 10.15 and Safari on a brand new 2020 MacBook Air
The display of any unicode character requires that your machine has a font installed which contains the glyph for it.  Apple doesn't provide fonts for all 130000 unicode characters.  For U+209b you need to download one.  A possible list is here.
And a font that can be used as a source for installing these glyphs is here: https://dejavu-fonts.github.io