putting a line break before the directive in the Gem (not suitable because I don't own the gem)

There's not much we can do there. It's not possible for a Svelte component to inspect another Svelte component and check if it exposes any prop

I understand this is not ideal, but sadly this is not something we can change as it's how Svelte works.

Just coming here to voice my agreement that these warnings are annoying and exist in other libraries as well. For me this happened with svelma. I didn't write the library code, so I don't have complete control over it even though I agree there is an argument to be had around whether I should be notified anyway. In either case, these warnings should be easily disabled since libraries don't always get updated over night.