MicroPython own readline implementation is superior now by providing
automatic indentation and completion (completion for GNU Readline was
never implemented). MICROPY_USE_READLINE=2 also wasn't build for a long
time and probably broken.
If GNU Readline is still beneficial for some cases, it can be achieved
with external wrappers like "rlwrap" (there will be the same level of
functionality, as again, there never was deep integration, like completion
support).