Why can't I see subscript "s" (Unicode Character “ₛ” (U+209B)) even though I can see lots of other unicode characters?
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