One can discuss 40k, but there may or may not be limits. Slaanesh is kind of lewd, and /tg/ is “work safe”, so will your posts be deleted if your deamonette pictures include titties? It depends on the mod, so really, who knows except for the mod that bans you? Then, with that in mind, what's the point in going out of your way to be creative and have fun? What if your picture trolls a mod somehow and you get banned?
Occasionally, when the mods are asleep, a fa/tg/uy might catch glimpses of threads talking about the old days on /tg/. The days when “/tg/ didn't need mods” because everything was really just kind of a joke, anyway. The days when even the trolls, however hated, did have a certain amount of bizarre class that at least made them interesting. To those not in the know, it may seem like oldfags grumping about nostalgia, but it honestly wasn't that long ago. Not only that, but those shenanigans that created all the fascinating stories here on 1d4chan weren't terribly rare in those days either.
Whatever they may say, don't believe them. /tg/ only has one rule under the mods: do whatever the mods want. There is no other persistently established set of rules. It's not your /tg/ anymore. It's not your gaming store anymore. You don't make the rules, and majority acceptance is meaningless. /tg/ belongs to the mods and thats sad.