Basically, there are no rules.
Just a few operating instructions for adults.
In game, keep it simple.
That includes guild chat, raids, group play, and how you deal with other players while wearing our tag. Sensitive topics stay out of the game chat. Partly because it reflects on the guild, partly because Blizzard is Blizzard.
Discord gives more room.
Some conversations are better kept there. That does not mean anything goes. If your idea of humour depends on making the place worse for others, you’re out.
Everything else:
Use common sense.