The validation messages are very non-descript and with the ability to set a closure callback, just "doesn't exist" in many cases might not be enough to get the right message across.
Potentially being able to either set on the fly the message optionally, or using some fluent way of specifying the message would make this far more usable from an end user's perspective.
(new ExistsEloquent(Post::class))->withMessage('Post is broke')
This would be useful for situations where potentially a relation check is used to confirm existence like:
new ExistsEloquent(Post::class, null, fn ($q) => $q->whereHas('comments'))->withMessage('Post is missing comments')
This is significantly clearer than just "This resource doesn't exist".