Because, thanks to the existence of the C++20 feature char8_t, this will work:
path p(u8"A/utf8/path");
u8path existed to allow the detection of the difference between a UTF-8 string and a narrow character string. But since C++20 will give us an actual type for that, it is no longer necessary.
What should I use in c++17?
Use u8path. Deprecation does not mean removed or inaccessible. It merely means subject to eventual removal.