First of all, I’m not sure if platform specific issues is the right tag since this is Linux only, so pls correct me if it is.
You can use certain unicode characters in Warthunder chat. For windows, it’s simply alt + numpad numbers.
Linux however works different. Linux uses a unicode code point (it looks like an underlined u) with ctrl + shift + u, and you then type the unicode code. it would look something like this u00b0 and pressing enter would result in °.
The issue is you can’t create a unicode code point in the Warthunder chatbox.