All symbols which are included in the fonts should be accessible by some TeX command. I wrote some code to compare the list of symbols (from symbols.js) against the fonts (as parsed by TTX, resulting in these files). Here are my findings:

• pund sterling sign £ (in Main_Italic font only)
• degree sign ° (see gagern@07131ec)
• various combining or modifier diacritics: acute, grave, tilde, dot, diaresis, macron, breve, ring, solidus, caron
• circumflex ^ and tilde ~ stand-alone
• em-dash — and en-dash – (will require changes to the lexer, I guess)
• typographic quotation marks “” and ‘’
• some misassociated typographic quotes at \u07e2 and \u07e3 in the Typewriter font
• division slash ∕? Not sure we need that, would be interesting to know how that came to be in the font, i.e. whether TeX is using this
• Maltese cross ✠ in AMS

I ignored some findings found by the script which sounded like something already covered by KaTeX, probably due to some other font using the semantically same symbol with a different code point. I also aggregated similar stuff quite freely, and in the process might have missed some important distinctions. I also ignored stuff which I know is already in progress, the dotless i and j in particular. So the above list is not guaranteed to be complete. Feel free to have a look at the raw output yourself and see whether you can spot anything I missed.