I’m probably doing exactly what they want here (e.g. having a conversation about it), but that letter is called “Eth” and was the Old English way of spelling the “th” sound: https://en.wikipedia.org/wiki/Eth
A number of linguistic buffs want to bring it back to the modern English alphabet.
I’ll never forget when the CDC’s official coronavirus FAQ had “Can I contract coronavirus from eating Chinese food?”. That was a real eye opener to just how many dumbasses are out there.