Doe anyone know? Thanks.

The rules don't specifically prohibit it.

ICORE Rule Book

"Any shooter whose firearm breaks the 180 during the course of fire will face disqualification. The 180 is an imaginary line separating up range from downrange. This line moves with the shooter throughout any course of fire."

However, many clubs just won't allow it by their own club rules.
Moreover, for a stage which requires you to move perpendicular to the backstop, while the muzzle of the gun in the cross-draw holster is pointed toward the spectators, I don't see how you draw from that holster without exposing the muzzle to the wrong side of the 180.

I guess you could do some sort of half-pirouette and then draw and fire.
I probably wouldn't let you do that on general principles.

Its a game.
Get a proper game holster, and just play the game - safely, and without antagonizing the S.O.

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WWSSD?
What would Skeeter do?

IPSC ROs are trained that a holster may not allow the muzzle to point further than 3 feet from the competitor's feet when standing relaxed. If the gun points further behind you when standing relaxed, you just broke the 180, in any game I've tried. I doubt any experienced RO would let you stand sideways to load and make ready.
I wouldn't, for safety reasons.

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Science plus Art

Ok, thanks.

I doubt that one would be allowed to use a crossdraw holster in ICORE, IDPA or USPSA. I have not seen a cross draw at any match in the last 15 years. I certainly would not allow it if I were the RO.