> we'd have to reimplement the UCRT function using the system API. 

Could the implementation drop support for os.O_TEXT? I think Python 3 should have removed both os.O_TEXT and os.O_BINARY. 3.x has no need for the C runtime's ANSI text mode, with its Ctrl+Z behavior inherited from MS-DOS. I'd prefer that os.open always used _O_BINARY and raised a ValueError if passed any of the C runtime's text modes, including _O_TEXT, _O_WTEXT, _O_U16TEXT, and _O_U8TEXT.

If _O_TEXT is supported, then we have to copy the C runtime's behavior, which truncates a Ctrl+Z from the end of the file if it's opened with read-write access. If Unicode text modes are supported, then we have to read the BOM, which can involve opening the file twice if the caller doesn't request read access.