True. I guess my point is that there isn't a statblock for it and you can only encounter an actual deity in their divine domain. But they effectively write the rules of that layer of their plane, or their demi-plane, so it's kind of wild to try to figure out what they can't do.
Tiamat might be the only one that's feasible because she lives on Avernus but doesn't rule it' she's imprisoned in hell. So she can probably only warp the area right around her. But even her aspect's stat block is CR 30 so who knows how you'd run her.
The problem here is that deities are bound by the plot, not the rules. If your DM tells you “no spell or weapon of mortal make could ever pierce the Queen of Dragons’ hide”, that means there is nothing you can do to hurt her. End of conversation. But, if your DM tells you “only a blade forged by the divine smith Watsisnaim could slay such a mighty foe”, it’s time for a fetch quest.