I did basic tests like logging in and simple GLX programs, video playing in mpv on GeForce 1050 Ti (nouveau support for those GPU series) with DoFlicky and all Nvidia packages uninstalled.
It is quite "intrusive" change so having feedback from users of AMD and Intel GPUs is mandatory.

Installing Nvidia drivers from repo via DoFlicky gave no errors and they are working fine. If those changes are appreciated I can prepare modifications for Nvidia packages as well.

Installing Nvidia drivers from repo via DoFlicky gave no errors and they are working fine. If those changes are appreciated I can prepare modifications for Nvidia packages as well.

There is absolutely no way the installation worked properly. LDM would need specialist support for glvnd migration.

In D1776#27301, @ikey wrote:

There is absolutely no way the installation worked properly. LDM would need specialist support for glvnd migration.

I couldn't believe it's that simple for mesa too ? and I should have mentioned before that removing Nvidia package requires libglvnd package to be reinstalled (until Nvidia package gets fixed for libglvnd).

Initially I ran into "mouse cursor on black background" issue at first but ln -s libGLX_mesa.so.0 $installdir/%libdir%/libGLX_indirect.so.0 fixed it:

Zrzut ekranu z 2017-12-26 22-22-22.png (1×1 px, 1 MB)

(disregard broken budgie, my .config was corrupted but everything is fine now)

Files list:
eopkg info -f mesalib
eopkg info -f mesalib-32bit

Steam is broken because it looks for libGL.so.1 and libEGL.so.1, they could be symlinked in libglvnd but I'm not sure if it's wanted behavior (of course they could be there for extended compatibility after transition).
I'll be happy to provide more proofs or details.

P.S.
I haven't tried moving /usr/lib/glx-provider/defaults/libglx.so provided by xorg-server to /usr/lib/xorg/modules/extensions/ yet as I don't know all changes made by Solus in other packages (LSI I'm thinking about you).