Hi all,
From July 23, any videos that were created as Unlisted prior to 2017 will be changed to Private visibility. Everything after this date will remain as Unlisted.
If anyone wishes for their form check videos etc., prior to this date to remain Unlisted, there is an opt-out link here: https://support.google.com/youtube/contact/older_unlisted_update
Alternatively you can still manually change them back after the change goes through.