Thanks for the heads up. I definitely want to opt out. I hope this place has some staying power, but I don't want an interconnectedness to flood the home feed with too much ragebait and garbage content anywhere near what reddit turned into.
Your home feed can default to "All," which shows content from all communities across all federated instances (not just Kbin and Lemmy) or it can default to Subscribed, which shows content only from communities you're subscribed to. Lemmy has a third option called "Local" which shows content from all local communities. Turning off federation for yourself in Kbin is similar to the local option in Lemmy, but allows you to still switch between All and Subscribed (ie, all local or subscribed local). Lemmy doesn't currently offer a "local subscribed" view.
The hamburger menu next to your name allows you to toggle between Subscribed and All. In the dropdown menu when you hover over your name, under Settings, you can select your default view (Subscribed vs All).
My personal preference is to default to subscribed and see content from across the Fediverse that I choose to see. This way I'm not missing out on any communities or magazines on federated instances that interest me. I sometimes switch to All with the Commented sort to see what the current zeitgeist is (where are the biggest conversations happening). I don't see much need to de-federate my view entirely since I can block communities/magazines, or entire domains, which means even under All I'm not bothered by content that I find annoying, offensive, etc.
In the dropdown menu when you hover over your name, under Settings, you can select your default view (Subscribed vs All).
Does that work for you? Mine is set to Subscribed but my home screen still defaults to All. From another thread, it sounds like at least one other person is having the same issue.
For what it's worth - on the microblogging (twitter-like) part of the fediverse, we've had pretty great results federating with lots of instances, but defederating from the ones that have problems with hate speech, spam, etc. It's led to a much more positive vibe than Twitter etc because people aren't so afraid of some random person misconstruing something they said and jumping down their throat about it, and there's no algorithm pushing rage "engagement" to the top.
One of the problems we do have in microblogging land, is the main mastodon developer has tried to direct new users to his own instance, making it by far the biggest mastodon instance, and then did a bad job controlling spam - so the other instances were left having to chose between cutting off a huge number of real users, or being inundated with spam. Having all the kbin and lemmy instances federate with each other, except where there are issues, can make the network overall more resilient against any one instance being mismanaged or invaded by spammers and trolls. The users on that instance will have to move, but there will be plenty of similar-sized instances ready to welcome them, and if the magazines are also spread out, they can keep posting to the same ones.
(I admit I also have a selfish interest here - I'm posting this from fedia,io, a different kbin instance. I joined through a different instance than the main one to help nudge things toward decentralization.)