Based on the core team communication, if you fork the project, you are persona non grata in Elm.

But that's the point. The community close to the core team is toxic, according to this post. So, there are incentives for the rest of the community to fork the project and have it their own way. There is no need to be in good terms with the core team.

Burning bridges is a risky approach if you are heavily invested in Elm.

I mean in normal communities it's OK to fork to experiment/support your use case but still participate in the mainstream community.

An act being allowed legally is distinct from an act being encouraged socially.