To be honest, you could do that in your bio.
The problem with implementing it as a field is that the overall UI isn’t designed for it, which means it’d have to be squeezed in somewhere in your settings, where people would forget to update it… and at that point, it wouldn’t be much more than an extra field under the bio.
Like, I totally get what you mean, but I don’t think there’s a good way to do it on a forum, or a very good idea. It’d have to be updated too often for what the UI makes quite comfortable.