I don't believe it has been, but it's an important distinction, one is simply clarifying that having had it explained to you that they cannot guarantee that the dish does not contain an allergen, you have chosen to order it anyway.
For example a place that only has one fryer and then someone wants something fried, but gluten free, and they have explained that the fryer is used for gluten products as well so there's a risk your food will have some cross contamination but you know that's fine for you, and tell them so, and they ask you to sign to show they've done their due diligence in telling you and you chose to order anyway.
I don't understand why anyone would take issue with that, unless of course they want to take the risk, but then want someone else to have the responsibility for it?
I've had people who have told me they need GF when our GF fryer is down and I have offered an alternative to the chips as cooking them in the other fryer will be cross contamination, for them to tell me that they're fine with that level but couldn't eat something else that comes on the dish and needs swapping for the GF version.
Customer knows their own allergy, who am I to tell them they're not allowed it? Isn't that bad customer service? Yet if they do decide to take the risk of the cross contamination, and I 'let' them, then I'm the one who takes the responsibility? Even having explained the risks?
Sorry if that's the case then I am not taking that risk and take poor customer service accusations over being sued!