This will need to be documented here: https://docs.godotengine.org/en/stable/tutorials/ui/bbcode_in_richtextlabel.html#reference

There's no standard bbcode for it (same is true for a some other tags we use).

Would [char=...] be more explicit to users maybe?

if there isn't a standard I would go with the full word [unicode=...] or with [char=...] like Akien suggested.
The short versions like [img] instead of [image] seem to only exists if there are html tags with the same name, if not, the full word is used (as far as I can tell, making it up tbh, so feel free to ignore).

Based on the RC discussion, let's go with [char=...] then.

Thanks!