Just to clarify, pygame_sdl2 is not the same thing as Ren'Py.Â
Just as Ren'Py < 6.99 was based on pygame, Ren'Py >= 6.99 is based on pygame_sdl2. Pygame_sdl2 is meant to be a mostly-compatible reimplementation of pygame, usable by other pygame applications and libraries.Â
pygame_sdl2 fundamentally supports Android and iOS, but parts of that support are in Ren'Py-branded tools. Removing that branding would be relatively easy.
When I say mostly-compatible, the only thing I think it makes sense to drop is support for < 32-bit surfaces, since doing so simplifies the code greatly. We could add support back, but doing means doing 4x the work whenever code touches the pixels.
There are a number of modules that haven't been implemented yet, mostly involving python's buffer support. And pygame_sdl2 hasn't been thoroughly tested outside of the parts of it used to support Ren'Py.Â