A summary of what is in this release since 1.12.0:

• Support for optional scalars in most languages.
• Rust Verifier, Object API and FlexBuffers support, and many API improvements.
• Overhaul of TypeScript support.
• gRPC for many languages brought up to date.
• C++ compile time field reflection.
• JS FlexBuffers support.
• Kotlin FlexBuffers and platform support.
• Swift Object API and many Swift performance improvements.
• Lua supports LuaJIT and many more fixes.
• Python FlexBuffers support.
• Many other fixes and smaller improvements, 371 commits since 1.12.0

Note, "2.0" doesn't signify any kind of major overhaul of FlatBuffers, it is merely trying to be more semver compatible, and this release does have breaking changes for some languages much like all releases before it.