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Is there a federation map somewhere so you can see what instances are federated with each other and which are not?
Yes.
https://federation-checker.vercel.app/
This link lets you type in an Instance name and it will tell you what other Instances allow it and which ones block it.
There's also this site, which gives you more options for filtering, but also seems to throw up errors pretty consistently in my experience.
In terms of figuring out which instances are cool with one another, this site is also quite useful. It's a web of trust model that a majority of major instances participate in.
It shows which instances endorse one another, with those instances obviously being federated. It also shows how instances choose to describe/tag themselves, which can give you a better understanding of their general vibe. And it also shows which instances are viewed with suspicion, as they are hesitated or censured by other instances.
@Martineski pinging you
I'd also be interested in knowing that.
Someone answered the comment above with a link, go check it out!