Ever since the Siege of Vraks Imperial Armour release, Forge World has stated that all of the models and rules contained therein would be official and standard. A reader over at Spikey Bits wrote in to Games Workshop and seems to have confirmed that yes, everything is valid in this brave new world.
It really shows how much things have changed in the last five years. Before MI40K was hacked and went down from 2012-2014, there was 15 or 16 codexes, and it was possible to know every single rule for every army. Now there are dozens of hardcover books, and even the huge Forge World catalog is a legal thing to bring to the table. Personally, I think that this is a good thing for hobbyists and a bad thing for tournaments. We are discussing running another MI40K tournament sometime soon, but figuring out the rules for it might be tough. On the other hand, having all these silky sweet models to build and gawk over is awesome. It’s a tough call on whether this is a positive change or not.