CocoaPods/Specs — pull request #6187 by fkazak (2013-12-16 09:24:18+00:00)
Title: Decreased GoogleProtobuf 2.4.1 deployment target from 7.0 to 5.0

protobuf itself doesn't depend on iOS SDK. I guess it should work with any iOS version/SDK, but I didn't check those prior to 5.0