Let's not sugar-coat this: do not tolerate fascists and bigots in your spaces. Period.

“communities existsting in their own spaces”

In some sense that is what defederating does, no?

I also have recently left Reddit. And I am all for blocking content that contains hate speech. Allowing fascist content to take root is unacceptable.

I am hoping that the new users are coming here with the intent to learn how this community works, before we try to remake the community we just left.

Yeah go and take a stroll through those instances and you'll see quickly why their posts aren't welcomed here.

Seems to be popular to defederate from right-leaning instances. The Fediverse basically started as a far-left stronghold, so it isn't surprising.

It has been common in other parts of the Fediverse - Mastodon for example.

I am hoping that the new users are coming here with the intent to learn how this community works, before we try to remake the community we just left.

I counter this part of your post by throwing in there that for me and my time on reddit, the worst parts of the broader experience were the fact that communities of neo-nazis (r/conservative, r/conspiracy), Donald Trump cultists (r/the Donald), incels (numerous subreddits including r/incels and r/theredpill), and pedophiles (r/just18 among other porn based subreddits that were quarantined and banned several years ago) were allowed their own communities on the platform for as long as they were. This gave these horrible ideas time to draw attention and build a userbase that then degraded the quality of reddit across multiple other communities.

If kbin or lemmyworld immediately start banning or defederating these instances or communities/magazines, then to me that is how this larger community works and it is inherently not former redditors migrating here to shape the Fediverse in the image of reddit.

Yes. Defederation of bad actors has always been common and it's expected.

Personally, I prefer individual users being empowered to easily block instances over instances blocking stuff "for" the users in most cases. Issues:

1. Users from other instances can still require mod actions. Moderation time is limited. Defederating from more problematic instances can be necessary if they cause more trouble than can be easily dealt with.

2. It is important for instance owners to achieve a coherent "front page" which includes the wider fediverse. I’m unsure if it is possible to ban individual instances from the frontpage while still allowing users to specifically visit them as they want.

3. Some instances are legally ambiguous or even contain content fully illegal in some countries.

I did for some research. Basically, all the top-defederated instances on this list are so for good reasons, often even legal reasons: https://fba.ryona.agency/scoreboard?blocked=100

Note that if you click on an instance, it will show you the various admin reasons for why people defederated.

The one I saw someone asking to be removed (exploding heads) seemed to be more normal discussion with a big extra dose of edgy humor magazines and swear words. This includes various slurs and straight up racism. This very much falls into the category of "I don’t want those here, but I’d prefer if users can still visit them" for me.

However, Lemmy.world admins have compiled some issues these users being, including harassing DMs to users. See for yourself: https://kbin.social/m/lemmyworld@lemmy.world/t/107898/Lemmy-world-Admin-Response-to-Defederation-from-Exploding-Heads#comments

Specifically, admins and community moderators of that instance were the problem. This seems like it would quickly fall into the "unfeasible to moderate on a case-by-case basis" category. Therefore, the nuclear option of defederation may be necessary.

The reason I am on lemmy.world is that so far, they have basically blocked nothing and that is the experience I'm looking for. I want to be the one who decides what I see.

That said, others prefer a more curated experience and thus choose other instances. That's the beauty of the fediverse, you can have both.