https://wiki.postmarketos.org/wiki/Nokia_N9_(nokia-n9)

That said, you may want to also consider Maemo Leste ... they haven't updated N9 support in a bit, but can definitely help you with it. With postmarketOS you'll likely be stuck to something like XFCE4 only due to non-working 3d acceleration. Leste has the acceleration working on N900 and Droid 4, and it's very likely they could help you get it up and running on N9 too

EDIT: Looks like 3d has worked on pmOS after all, at some point, https://twitter.com/fi1ippz/status/9...194349056?s=09.
Leaving the previous text intact though, since you'll likely still get better accel support on Leste than anywhere else.