Sometimes the best software is no software. You can do this without some emojified github readme, the text editor your messenger uses will be just fine.

(If you're not a beginner it's probably fine to just use "algebraic" notation anyway.)

I thought it would at least read the slack websocket and send slack messages. Having to copy paste is horrible.

It's not very hard, you can take inspiration from my project "localslackirc", it even has a tool to extract the slack token from firefox's cookies (if you copy paste code, keep in mind it's GPL licensed).

en passant doesn't work in the demo

I used to play chess over email that way, back in 1994, using a plugin for the Thor email client on Amiga