They don't support it? That's weird (maybe a missing feature) given that it's quite easy to add to anything that has signed metadata, see e.g. this for OpenPGP: https://github.com/wiktor-k/openpgp-proofs#openpgp-proofs

I'm not very familiar with the service, but AFAIK they don't. It would be great if they added it.

EDIT: It looks like it might, from the front page, I will try it out to make sure. If it does, that'll be great!

EDIT 2: It sort of does, but it's on a per-key basis, not an entire identity. You can publish proof on Twitter/Github/whatever, but it's only for one specific key, and it's one key per service, which means you can't only have one identity and multiple services.