We are very close to the point where people can credibly insist that a real video is a deepfake. But so far I've only seen realistic deepfakes where the face is swapped.
When a deepfake of someone actually doing bad things is possible, we will see people claiming the video is fake.
When this is trvial to do, I'm not sure what happens to the concept of security cameras. The recordings may need to be hashed and "notorized" on a blockchain to be legally valid.