What is the state on Third Party parts?

[Eugen]

I think its more about directly compatible parts made as an addition to Lego
(although sometimes the official lego parts are made for them)
example (base part is original lego, electric connections are for PU):
https://library.ldraw.org/tracker/34978

or the FXtracks which fit the old 9V tracks...

Yes, I'm aware of that manufacturers, which are producing LEGO compatible addition parts, like «Brickmania®» (custom addition parts for scale model sets of military vehicles and custom minifigs).

But also, there are some small teams and individuals producing addition parts (3D-printed ready to ship, to be 3D-printed on demand, or to be printed by users itself). And there are addition parts for 3D-printing shared for free (including under CC BY).

I read the whole thread on Third Party parts inclusion into LDraw Library: https://forums.ldraw.org/thread-21841.html

But yet, its unclear. It might be good to compile a "List of acceptible Third Party parts manufacturers" (as «Cobi» custom parts are blacklisted, and «S-Brick» custom parts are acceptible, need to know which else are blacklisted and acceptible).

No. Virtual only files are not eligible for inclusion.

What about Printabrick.com and similar homebrew individuals? Which producing (or produced in the past) in a small number by on demand 3D-printing.