Bridgy Fed, a bridge between the Fediverse and other protocols such as BlueSky, is using an opt-out model and that raises a lot of discussion

You can't defederate from the bridge because it's not going to be the only instance of the bridge. Anyone will be able to host an instance of the bridge server, just like anyone can host an instance of any fedi software. Sure, you can block brid.gy, but then you also have to block every other instance, too. On the mastodon instance I use, there are 45 blocked instances of Birdsite Live, a (now defunct, one way) Twitter bridge!

Opting out with a hashtag technically works, but there is a character limit in the mastodon bio. It also depends on all bridges agreeing to the same hashtag.

Opt-in just makes a lot more sense, imo. It avoids different instances hosting duplicate mirrors and it avoids anyone (on bsky or fedi!) from having their posts scraped and mirrored to a different network without their knowledge.

Instance can block bridge domain and it will not be federated

I was referring to the

Put the text #nobridge in your profile bio, refresh your profile on your user page, and Bridgy Fed will stop bridging your account. Or feel free to send me a request privately.

https://fed.brid.gy/docs#opt-out

Seems like defederation is not enough in this case, as it's not mentioned as a way to opt-out.