I can't see such a place making an exception for the Decurion. It's made of nothing but Formations.
If someone disallows formations in the age of 7th edition they may as well be saying CAD and allied detachments only, Formations "are" detachments and Decurion is a formation detachment not a FOC detachment
No, reread the rules for the Decurion. At no point does it say it is a Formation. It only references it as a Detachment. It is made up of Formations and units, true, but they are organized by a force organization chart. At no point in the rules for an FOC Detachment does it ever state that it is only made up of Roles.
Note, this only really matters in situations where a tournament is limiting the force to non-Formations or someone with their head up their arse won't recognize Formations as legal ways to build an army.
If a TO is restricting a Tournament to no formations Decurion isn't an option anyhow since the core requirement is "yup" a formation. I don't see you disagree with the core principal that all formations are detachments.
I'll give you this, since Decurion is not assigned a page with a formation symbol it is not itself a formation which would technically make it a unique form of detachment, never mind that how it is structured is no different than every other formation made up of multiple formations, my original point was that the Decurion detachment is more of a Formation than a FOC detachment(which selects its units based off battlefield role.)
As far as tournaments go, frankly the FOC based detachments are hardly more balanced than the formations so restricting them is just a lazy way of avoiding changes to the meta. Be better off just restricting armies to one faction and making each faction play the same faction if you want balance.
Depends what you mean by changes to the Meta
GW has tried hard to push super heavies down from Apoc only but on the other hand provided a push from the bottom with everything scoring and progressive missions.
TO's allowing super heavies, forgeworld & D weapons generally excludes a huge number of 40k players so what you get is a hardcore only attendance which at the moment isn't enough to keep the game alive competitively in our area anyway.
We have a lot of fun playing 40k but its all done via negotiation otherwise your ending up with very 1 sided games which are just boring and that's all down to the 7th changes the supplements formations etc etc.