My understanding is that proto3 no longer allows you to detect field presence and no longer supports non-zero default values because this makes it easier to implement protobufs in terms of “plain old structs” in various languages, without the need to generate accessor methods. This is perceived as making Protobuf easier to use in those languages.
(I personally think that languages which lack accessors and properties aren’t very good languages and protobuf should not design down to them, but it’s not my project anymore.)