I don't really care what FW or GW declares "legal" anymore. They say the Dataslates are "official" but there's really no good reason for them existing (or at least the Tau Firebase one). Basically, the value of GW's word with me over the last two weeks has completely evaporated.
I will also point out that main GW pays zero attention to what FW does, FW isn't even allowed at Throne of skulls tournaments. (though I hear Escalation might be now? WTF?)
If you just want to push models around and roll dice for the sake of rolling dice, hey it probably doesn't matter much what rules you use. "Forge the narrative!"
If you want a competitive game where the skills of the player actually matter, hey you're probably going to have to re-write some rules. Ban some things, allow others. This has really been true for a while but it's super-obvious now.