The core Protocol Buffers technology is provided courtesy of Google.
At the time of writing, this is released under the BSD license.
Full details can be found here:

https://github.com/protocolbuffers/protobuf

The .NET implementation is Copyright 2008 Marc Gravell
At the time of writing, this is released under the Apache License, Version 2.0
Full details can be found here:

https://github.com/protobuf-net/protobuf-net

This Unity implementation is Copyright 2019 Pavel Koshelev

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.