C++

• Make serialization method naming consistent
• Make proto runtime + generated code free of deprecation warnings
• Moved ShutdownProtobufLibrary() to message_lite.h. For backward compatibility a declaration is still available in stubs/common.h, but users should prefer message_lite.h
• Removed non-namespace macro EXPECT_OK()
• Removed mathlimits.h from stubs in favor of using std::numeric_limits from C++11
• Fixed bug in parser when ending on a group tag
• Add a helper function to UnknownFieldSet to deal with the changing return value of message::unknown_fields()
• Fix incorrect use of string_view iterators
• Support direct pickling of nested messages
• Skip extension tag validation for MessageSet if unknown dependencies are allowed
• Updated deprecation macros to annotate deprecated code (#6612)
• Remove conversion warning in MapEntryFuncs::ByteSizeLong (#6766)
• Revert "Make shared libraries be able to link to MSVC static runtime libraries, so that VC runtime is not required." (#6914)

Java

• Remove the usage of MethodHandle, so that Android users prior to API version 26 can use protobuf-java
• Publish ProGuard config for javalite
• Fix for StrictMode disk read violation in ExtensionRegistryLite
• Include part of the ByteString's content in its toString().
• Include unknown fields when merging proto3 messages in Java lite builders

Python

• Add float_precision option in json format printer
• Optionally print bytes fields as messages in unknown fields, if possible
• FieldPath: fix testing IsSet on root path ''
• Experimental code gen (fast import protobuf module) which only work with cpp generated code linked in

JavaScript

• Remove guard for Symbol iterator for jspb.Map

PHP

• Avoid too much overhead in layout_init (#6716)
• Lazily Create Singular Wrapper Message (#6833)
• Implement lazy loading of php class for proto messages (#6911)

Ruby

• Ruby lazy wrappers optimization (#6797)

C#

• (RepeatedField): Capacity property to resize the internal array (#6530)
• Experimental proto2 support is now officially available (#4642, #5183, #5350, #5936)
• Getting started doc: https://github.com/protocolbuffers/protobuf/blob/master/docs/csharp/proto2.md
• Add length checks to ExtensionCollection (#6759)
• Optimize parsing of some primitive and wrapper types (#6843)
• Use 3 parameter Encoding.GetString for default string values (#6828)
• Change _Extensions property to normal body rather than expression (#6856)

Objective C

• Fixed unaligned reads for 32bit arm with newer Xcode versions (#6678)