I could understand an App no longer being supported such as major version iOS update. But off load feels like to be a new blacklist with new iOS update.
At what point will Apple "off load" my contacts, photos or videos?
To your last question: Contacts don't take much space, so probably never.
Photos and videos, if you use iCloud sync, you can choose to hold lower or original quality on your device but either way iCloud is the authoritative storage point. I don't think they get offloaded like apps when you need more space but they're big enough for that to be realistic. Conversely I think when your local storage is full, new photos taken elsewhere won't sync down.
Installing an OS update would need temporary storage space on the phone, which could trigger the offload process if the storage space is low. Then the app wouldn’t be able to be downloaded again if it was removed from the store.