Hello! PostmarketOS contributor here.
You're right, postmarketOS is not as free as Trisquel or Replicant at the
moment. Most of the devices require proprietary firmware for basic
functionality. Since we would not like to limit support to a small number of
phones, that work without non-free software we do include some proprietary
software in postmarketOS.
Even though we're not aiming to get on the Free Software Foundation's list of
free GNU distributions, we could implement a feature in pmbootstrap to build
a fully libre version of postmarketOS, and leave out the non-free parts on
the few devices, that are able to function without non-free software, even if
that means losing functionality.
If anyone is interested in helping postmarketOS, or making it more freedom
friendly, come and talk to us!
#postmarketos on Freenode
#postmarketOS:disroot.org (Matrix)