This is why especially the IndieWeb people are adamant about that everyone should be their own identity provider on the net and not give that role away to any silo|3rd party. Yes, a domain costs (just like a phone number). Even if it just holds a single page referencing “official” profiles. I’m aware that this isn’t an identity check but it’s enough to tell profile equivalence and this is the more important part.
BTW: Love your books 🤓