AFAIK ignoring unknown fields is a MUST in the protobuf spec. It's safe to assume all of their tooling is built around this. It wouldn't make a difference anyway. As vitus pointed out, deleting the field isn't much harder than changing it to an unknown field.

You can do cert pinning. And the user can modify the app to pin their own cert. And you can lock down the device so the user can't modify the app. And the user can get a different device where they can modifiable apps. And you can add device attestation. And it's not yet feasible to extract an attestation key from a device, but it probably will be in the future. And then you will switch it to a physically uncloneable function. And then someone will figure out how to physically clone it anyway. And so on.

The war on ad-blocking is fundamentally the war on general-purpose computing. By the time you achieve unskippable ad blocking,

You know, you could also just refuse to send any video segments until the time when the ad is supposed to be over. Then the user may try to download their videos in advance, but most of the time they don't know what they're going to watch that far in advance, so they'll sit through the ad to avoid sitting through a black screen. That seems like a more sane thing to try. And you don't have to destroy the fabric of society to do it.