Note: The author (Amanda Kornoushenko aka Lyberta) prepared a great proposal P2146, but declared loss of interest in the progress. Also post on reddit and repository on githib were deleted. Fortunately link to code archive has survived (not sure at which revision of proposal), so here just raw content of this archive. I think this project can be used as a base for a great binary io library that can work on top of c++17 or even less standard.
IANAL, but the text of BSL says that "Permission is hereby granted... to use, reproduce, display, distribute, execute, and transmit the Software, and to prepare derivative works of the Software" so I'm sure that according to the license I'm allowed to publish this copy of the code even if original work is removed by the author.

Implementation of P2146 - Modern std::byte stream IO for C++

Reference implementation of P2146 - Modern std::byte stream IO for C++.

None of the major compilers support all of the C++20 features required to implement this proposal. The missing features are:

GCC: std::bit_cast.
Clang: concepts library, ranges, std::bit_cast.
MSVC: ranges.

Component	Implementation status
std::io::make_error_code	✅
std::io::make_error_condition	✅
std::io::category	Partial
std::io::io_error	Partial
std::io::offset	✅
std::io::position	✅
std::io::input_stream	✅
std::io::output_stream	✅
std::io::stream	✅
std::io::input_output_stream	✅
std::io::seekable_stream	✅
std::io::buffered_stream	✅
std::io::read_raw	✅
std::io::write_raw	✅
std::io::format	✅
std::io::context	✅
std::io::input_context	✅
std::io::output_context	✅
std::io::default_context	✅
std::io::read	Partial
std::io::write	Partial
std::io::readable_from	✅
std::io::writable_to	✅
std::io::any_input_stream	✅
std::io::any_output_stream	✅
std::io::any_input_output_stream	✅
std::io::in	Partial
std::io::out	Partial
std::io::err	Partial
std::io::input_span_stream	✅
std::io::output_span_stream	✅
std::io::input_output_span_stream	✅
std::io::basic_input_memory_stream	✅
std::io::basic_output_memory_stream	✅
std::io::basic_input_output_memory_stream	✅
std::io::input_file_stream	Partial
std::io::output_file_stream	Partial
std::io::input_output_file_stream	Partial

Notes on floating point support

std::io::floating_point_format::native is supported everywhere.

Full std::io::floating_point_format::iec559 functionality requires mainstream endianness, native ISO 60559 types and lack of padding bits.

Only binary32 and binary64 formats without padding bits are supported right now.
Assuming the previous point, std::endian::native is supported regardless of its value.
Otherwise, only byte swaps are supported (little to big, big to little). This requires std::endian::native to be either std::endian::little or std::endian::big.

Notes on file IO and standard stream objects

Only POSIX and Windows are supported right now.
If a POSIX or Windows call produces the error that is not in the proposal, std::system_error is thrown.

POSIX specific notes

std::io::creation::always_new does not have direct equivalent in POSIX and is implemented using std::filesystem which suffers from TOCTOU.

Windows specific notes

File buffer size is hardcoded to 4096.
Standard stream objects are not thread-safe yet.

How to build

General build dependencies:

Git
CMake 3.12 or newer

Linux

Step 1: Create build directory and change into it

mkdir Build
cd Build

Step 2: Configure CMake with your choice of compiler and standard library

GCC and libstdc++

Requires at least GCC 10.2.

Assuming g++-trunk is not default compiler but it is the latest version you want:

cmake -D CMAKE_CXX_COMPILER=g++-trunk ..

Clang and libstdc++

Requires at least Clang 10 and libstdc++ shipped with GCC 10.2.

Assuming clang++ is not default compiler but it is the latest version you want:

cmake -D CMAKE_CXX_COMPILER=clang++ ..

Clang and libc++

Not usable right now because of lack of concepts library.

Assuming clang++ is not default compiler but it is the latest version you want:

cmake -D CMAKE_CXX_COMPILER=clang++ -D CMAKE_CXX_FLAGS=-stdlib=libc++ ..

Step 3: Build the library

cmake --build .

Windows

Mingw-w64 on Linux (cross-compiling)