Even if a process re-serializes a message, unknown fields will be preserved, if using the official protobuf libraries proto2 or 3.5 and later. Only in 3.0 did they drop unknown fields, which was complete lunacy. That decision was reverted for proto 3.5.

Some of the APIs (C++ for example) provide methods to access unknown fields, in case they were only mostly unknown.

I do remember some debate around required/optional in proto2, and how the consensus was to not have required in proto3 - parties had good arguments on both sides, but I think ultimately the backward-forward compatibility argument won. With required, you can never retire a field, and older service/client would not work correctly. I haven't used proto2 since then, been using proto3 - but was not aware of what another poster here mentioned about proto 3.5 - so now have to read...