I was there at the time too and couldn't believe people were defending required. I had already run into enough problems with it that I stopped using it earlier, and I worked on a relatively small system with not that many moving parts! I can hardly imagine how much pain required must have caused in something like the indexing pipeline.

The thing I realized was that even if a field is present, you almost always still want to do further checks on it before declaring the whole thing valid and good to go. "required" just tells you that it's an integer, for example, it doesn't tell you if it's in the range [0, 1000). Or that a string is a valid filesystem path. Or that two parallel arrays are the same size. You have to do those things anyway, so the has_... check is no big deal. And making everything optional is really nice when you have to make large changes to your protocol and don't want to have to carry around dummy values forever.

Protocol buffers weren't designed from scratch by Jeff Dean, they evolved from various ad-hoc ways of specifying query parameters (which, ok, were probably mostly written by Jeff), so I don't think it reflects badly on him to say that required was a mistake. Groups were a mistake in retrospect too, but no one thinks that means Jeff is an idiot.