Usually, they don't break anything fundamental. But they do break things all the time.

Every new event kind can't be seen in Damus until they code it. In Amethyst, it shows as "Post not found". We don't remove the post. It's just the way Nostr works. Anyone can create these new event kinds, with or without NIPs. And when that happens, things "break" (wrong word, though. They don't break, people just can't see it for a while)

This happened when Markdown was implemented. Suddenly many clients, Amethyst included, couldn't display the content correctly. More recently we had mathematical equations being added to Habla News. Those don't render in Amethyst as well. Similar to Private Zaps (which is not even a NIP) that Amethyst users couldn't see it until last Monday.

This is a normal course of business for any Nostr client.

Refusing to implement a NIP and creating a hacked-up version of the same NIP, however, is something new.