OK, I hoped that registration at Microsoft’s GitHub would not be necessary here. Tried to register at GitHub using Whonix, without giving away my personal data. No success, despite correctly solving 10 (!) torturous anti-robot puzzles:
More attempts didn’t help either. I’m afraid I’m not ready to reveal my identity for this account to the world. (However, revealing it to the Qubes team might be possible, if it helps.