Ah right. Because I think you could build it into a frontend or app and anyone using that app would be able to see a feed of joined up communities.
I’m not sure how a different implementation would work. There is a lot of public/private key stuff happening in ActivityPub so you can be sure that the federated posts are actually coming from the community you subscribe to and the users it claims wrote them.
If you work it out I’d be interested to see!
FYI, a Lemmy dev confirmed this setting doesn’t actually exist in the database anymore. A relic of times gone by, and it’s going to be removed in the next version.