ActivityPods are being developed. From their website:

What is the main shortcoming of ActivityPub ?

ActivityPub wants to make it possible to create decentralized apps. But to post videos, you need an account on a PeerTube instance. And to post images, you need an account on a PixelFed instance. You must thus handle multiple accounts, with their profile, list of followers, etc.

How does ActivityPods solve this shortcoming ?

With ActivityPods, you have only one profile, one outbox, one inbox and one list of followers - all in a single place. Applications connect to your Pod to post activities, read the inbox and fetch data. And of course they can connect to any existing fediverse application !

There is an identity protocol for achieving that, called zot. However it requires the forums/sites you're visiting to have Zot implemented. That is likely not the case, as Zot is way less prevalent than even ActivityPub.

There is OpenID, it was all the rage about a decade ago

Isn't OIDC basically what you want? You just need to convince the forums to use it.

@haui_lemmy
That's look like federated SSO to me.
@opensource