They said it will expire on 31st December 2019. Must have done it earlier than the Itch one.
I've just looked it up and they seem to last 3 years (unless there is a change of circumstances).
https://www.irs.gov/instructions/iw8ben#idm140376280590176
Thanks for the info. I’m guessing the third party we use will contact you about updating it when the time comes.