How many characters can notifications have on iOS?

I'm writing guidelines for drafting notifications. Is there a hard number how many characters can be in a notification?

I'm especially interested in compatibility with older iOS versions: Which limit should writers respect to ensure compatibility with iOS 9?

Another point of interest: Notifications can have a title. Is the title backwards compatible to older iOS versions as well?

Finally, is there data on what length of notifications people engange best with?