Is it arrogant or a clear and straightforward announcement that a Decision has been made and these are the consequences? I'm not seeing any arrogance in the message myself.

How is this arrogant? Are open source developers now responsible for ensuring every fork works with the dependencies and changes they make?

This seems like a long window, given to ports to say, "we are making changes that may impact you, heads up." The options presented are, frankly, the two primary options "add the dependency or tell people you are no longer a current port".

"Arrogant" does not mean "forceful" or "assertive" or "makes me angry".

This is forceful, assertive, and probably makes people angry.

Does the speaker have the authority to make this happen? Because if so, this is just a mandate and it's hard to find some kind of moral failing with a change in development direction communicated clearly.