Decentralized IDs could be to social apps what Metamask is to Defi apps: one-click login across many applications, without a centralized identity provider like Twitter or Facebook. Enabling a much bigger ecosystem.

Yes indeed. It's called Indieauth indieauth.spec.indieweb.org/

What have been the barriers to adoption? Needing users to own their own domains and set up authorization endpoints, requiring some technical ability?

That's part of it, though the RelMeAuth model can mitigate that to some extent. A lot of it is having a use case to authorise for. Micropub is one use case that can make sense to users

Nah, don’t forget that every micro.blog account is an IndieAuth account too. Users don’t need to have any knowledge of anything under the hood for that to work. We need more service providers to implement it more than anything.

That's very true, and the Wordpress plugin makes the case as well.