It’s a feature but not the best practice if the idea would be forums (and users) being free of domains
I don’t think the idea is for users to be free of domains. One of the key benefits of tying users to their instance is that you defederate from the users of an instance when you defederate from an instance. If users were not bound to instances, it would be hard to defederate from certain groups without manually defederating a million users. Users being tied to domains makes moderation via defederation much, much simpler.
The design approach of Lemmy however, speaks “hegemony” all over. It says a lot about the mindset of its creators.
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ActivityPub, AFAIK only defines a protocol for communicating datasets between instances, not the structures in which federation should be done.
I’m not an expert on ActivityPub but I think you’re wrong about this being Lemmy’s design decision. I think ActivityPub is designed in this way and it is intentional. I mean, all other ActivityPub apps do the same thing (e.g. Mastodon users are also tied to their instance).
forums being bound to domain names instead of them having Universal IDs thus being agnostic of which node they are actually hosted on.
Just want to point out that domain names are also perfectly capable of being agnostic about nodes - i.e. you can host multiple websites on a single computer or distribute the hosting of a website across many computers. I’m not really sure what you’re saying here but I don’t know if it’s important.