C++

• Optimize and simplify implementation of RepeatedPtrFieldBase
• Don't create unnecessary unknown field sets.
• Remove branch from accessors to repeated field element array.
• Added delimited parse and serialize util.
• Reduce size by not emitting constants for fieldnumbers
• Fix a bug when comparing finite and infinite field values with explicit tolerances.
• TextFormat::Parser should use a custom Finder to look up extensions by number if one is provided.
• Add MessageLite::Utf8DebugString() to make MessageLite more compatible with Message.
• Fail fast for better performance in DescriptorPool::FindExtensionByNumber() if descriptor has no defined extensions.
• Adding the file name to help debug colliding extensions
• Added FieldDescriptor::PrintableNameForExtension() and DescriptorPool::FindExtensionByPrintableName().
  The latter will replace Reflection::FindKnownExtensionByName().
• Replace NULL with nullptr
• Created a new Add method in repeated field that allows adding a range of elements all at once.
• Enabled enum name-to-value mapping functions for C++ lite
• Avoid dynamic initialization in descriptor.proto generated code
• Move stream functions to MessageLite from Message.
• Move all zero_copy_stream functionality to io_lite.
• Do not create array of matched fields for simple repeated fields
• Enabling silent mode by default to reduce make compilation noise. (#6237)

Java

• Expose TextFormat.Printer and make it configurable. Deprecate the static methods.
• Library for constructing google.protobuf.Struct and google.protobuf.Value
• Make OneofDescriptor extend GenericDescriptor.
• Expose streamingness of service methods from MethodDescriptor.
• Fix a bug where TextFormat fails to parse Any filed with > 1 embedded message sub-fields.
• Establish consistent JsonFormat behavior for nulls in oneofs, regardless of order.
• Update GSON version to 3.8.5. (#6268)
• Add protobuf_java_lite Bazel target. (#6177)

Python

• Change implementation of Name() for enums that allow aliases in proto2 in Python
  to be in line with claims in C++ implementation (to return first value).
• Explicitly say what field cannot be set when the new value fails a type check.
• Duplicate register in descriptor pool will raise errors
• Add slots to all well_known_types classes, custom attributes are not allowed anymore.
• text_format only present 8 valid digits for float fields by default

JavaScript

• Add Oneof enum to the list of goog.provide

PHP

• Rename get/setXXXValue to get/setXXXWrapper. (#6295)

Ruby

• Remove to_hash methods. (#6166)