I believe it would be more aligned with the C++ philosophy that this newly standardized feature be as fundamental as possible - as close to the metal as possible - and thus be a byte-swap proposal and not a "make-it-this-endian" proposal. "make-it-this-endian" is built from two lower level primitives: is_little_endian? and byteswap. To standardize "make-it-this-endian" would leave the use cases needing simply byteswap out in the non-standardized cold. And yet it would be easy to _build_ "make-it-this-endian" on top of is_little_endian? (now called `std::endian` in C++20) and byteswap.
(BTW, and off topic, I see that `std::endian` doesn't support the VAX ... should it have been a _template_, like `std::endian<type>` so that integers and floats could have different endianness?)
-- David