Ruby

• Dropped Ruby 2.3 and 2.4 support for CI and releases. (#9311)
• Message.decode/encode: Add max_recursion_depth option (#9218)
• Rename max_recursion_depth to recursion_limit (#9486)
• Fix conversion of singleton classes in Ruby (#9342)
• Suppress warning for intentional circular require (#9556)
• JSON will now output shorter strings for double and float fields when possible without losing precision.
• Encoding and decoding of binary format will now work properly on big-endian systems.
• UTF-8 verification was fixed to properly reject surrogate code points.
• Unknown enums for proto2 protos now properly implement proto2's behavior of putting such values in unknown fields.

Java

• Revert "Standardize on Array copyOf" (#9400)
• Resolve more java field accessor name conflicts (#8198)
• Don't support map fields in DynamicMessage.Builder.{getFieldBuilder,getRepeatedFieldBuilder}
• Fix parseFrom to only throw InvalidProtocolBufferException
• InvalidProtocolBufferException now allows arbitrary wrapped Exception types.
• Fix bug in FieldSet.Builder.mergeFrom
• Flush CodedOutputStream also flushes underlying OutputStream
• When oneof case is the same and the field type is Message, merge the subfield. (previously it was replaced.)’
• Add @CheckReturnValue to some protobuf types
• Report original exceptions when parsing JSON
• Add more info to @deprecated javadoc for set/get/has methods
• Fix initialization bug in doc comment line numbers
• Fix comments for message set wire format.

Kotlin

• Add test scope to kotlin-test for protobuf-kotlin-lite (#9518)
• Add orNull extensions for optional message fields.
• Add orNull extensions to all proto3 message fields.

Python

• Dropped support for Python < 3.7 (#9480)
• Protoc is now able to generate python stubs (.pyi) with --pyi_out
• Pin multibuild scripts to get manylinux1 wheels back (#9216)
• Fix type annotations of some Duration and Timestamp methods.
• Repeated field containers are now generic in field types and could be used in type annotations.
• Protobuf python generated codes are simplified. Descriptors and message classes' definitions are now dynamic created in internal/builder.py. Insertion Points for messages classes are discarded.
• has_presence is added for FieldDescriptor in python
• Loosen indexing type requirements to allow valid index() implementations rather than only PyLongObjects.
• Fix the deepcopy bug caused by not copying message_listener.
• Added python JSON parse recursion limit (default 100)
• Path info is added for python JSON parse errors
• Pure python repeated scalar fields will not able to pickle. Convert to list first.
• Timestamp.ToDatetime() now accepts an optional tzinfo parameter. If specified, the function returns a timezone-aware datetime in the given time zone. If omitted or None, the function returns a timezone-naive UTC datetime (as previously).
• Adds client_streaming and server_streaming fields to MethodDescriptor.
• Add "ensure_ascii" parameter to json_format.MessageToJson. This allows smaller JSON serializations with UTF-8 or other non-ASCII encodings.
• Added experimental support for directly assigning numpy scalars and array.
• Improve the calculation of public_dependencies in DescriptorPool.
• [Breaking Change] Disallow setting fields to numpy singleton arrays or repeated fields to numpy multi-dimensional arrays. Numpy arrays should be indexed or flattened explicitly before assignment.

Compiler

• Migrate IsDefault(const std::string*) and UnsafeSetDefault(const std::string*)
• Implement strong qualified tags for TaggedPtr
• Rework allocations to power-of-two byte sizes.
• Migrate IsDefault(const std::string*) and UnsafeSetDefault(const std::string*)
• Implement strong qualified tags for TaggedPtr
• Make TaggedPtr Set...() calls explicitly spell out the content type.
• Check for parsing error before verifying UTF8.
• Enforce a maximum message nesting limit of 32 in the descriptor builder to guard against stack overflows
• Fixed bugs in operators for RepeatedPtrIterator
• Assert a maximum map alignment for allocated values
• Fix proto1 group extension protodb parsing error
• Do not log/report the same descriptor symbol multiple times if it contains more than one invalid character.
• Add UnknownFieldSet::SerializeToString and SerializeToCodedStream.
• Remove explicit default pointers and deprecated API from protocol compiler

Arenas

• Change Repeated*Field to reuse memory when using arenas.
• Implements pbarenaz for profiling proto arenas
• Introduce CreateString() and CreateArenaString() for cleaner semantics
• Fix unreferenced parameter for MSVC builds
• Add UnsafeSetAllocated to be used for one-of string fields.
• Make Arena::AllocateAligned() a public function.
• Determine if ArenaDtor related code generation is necessary in one place.
• Implement on demand register ArenaDtor for InlinedStringField

C++

• Enable testing via CTest (#8737)
• Add option to use external GTest in CMake (#8736)
• CMake: Set correct sonames for libprotobuf-lite.so and libprotoc.so (#8635) (#9529)
• Add cmake option protobuf_INSTALL to not install files (#7123)
• CMake: Allow custom plugin options e.g. to generate mocks (#9105)
• CMake: Use linker version scripts (#9545)
• Manually *struct Cord fields to work better with arenas.
• Manually destruct map fields.
• Generate narrower code
• Fix https://github.com/protocolbuffers/protobuf/issues/9378 by removing shadowed cached_size field
• Remove GetPointer() and explicit nullptr defaults.
• Add proto_h flag for speeding up large builds
• Add missing overload for reference wrapped fields.
• Add MergedDescriptorDatabase::FindAllFileNames()
• RepeatedField now defines an iterator type instead of using a pointer.
• Remove obsolete macros GOOGLE_PROTOBUF_HAS_ONEOF and GOOGLE_PROTOBUF_HAS_ARENAS.

PHP

• Fix: add missing reserved classnames (#9458)

C#

• Fix trim warnings (#9182)
• Fixes NullReferenceException when accessing FieldDescriptor.IsPacked (#9430)
• Add ToProto() method to all descriptor classes (#9426)
• Add an option to preserve proto names in JsonFormatter (#6307)

Objective-C

• Add prefix_to_proto_package_mappings_path option. (#9498)
• Rename proto_package_to_prefix_mappings_path to package_to_prefix_mappings_path. (#9552)
• Add a generation option to control use of forward declarations in headers. (#9568)