We have had a few clients of our UNA.IO Servers complain that they could no longer find Safari ID within OneSignal. After talking to OneSignal we have found they have integrated Safari notifications into their Web App. This means you do not need to set up Safari ID separately.
Just use the OneSignal App ID as you would normally and ignore Safari ID.
Maybe UNA.IO team need to remove the "Apple Safari Web ID on the Web Push settings" field to stop confusion.