> These rules cover most religions and most lifestyle diets; these do exclude wishy-washy bullshit.

What about people who only want to eat free-range eggs (and not any products derived from eggs from caged hens?); or do not want to eat fish from non-sustainable fishing? Sure, they can stick to the vegan diet, but isn't it discriminatory that their specific preferences are not taken into account, and that they are mixed with a larger group? (In the same way that, I guess, you would not find it acceptable if people who want to eat halal or kosher were directed to eat vegan food; although note that kosher can be more restrictive than vegan due to requirements on checking for possible insect infestation of certain vegetables.)

What about people who refuse to eat GM food? People who refuse to eat food that wasn't produced locally, because they want to cut down on pollution? What about fruitarians? (https://en.wikipedia.org/wiki/Fruitarianism) Jain vegetarians? (https://en.wikipedia.org/wiki/Jain_vegetarianism ; note that it excludes onions so vegan diet is not OK). Sarbloh bibek for Sikhs? (https://en.wikipedia.org/wiki/Diet_in_Sikhism#Sarbloh_Bibek)

Who is to make a decision about which practices are "bullshit" and which ones are "serious"? or whether one was "sufficiently" committed to a practice in the past? If you specifically accommodate some of these practices but not others, you are discriminating against people following practices which are too rare or not sufficiently established. In this spirit I think your original list would be discriminatory relative to the other practices I have listed above.

My claim is that a prison has to manage the practical life of the inmates and feed them (with a default option; I do not object them buying different food items from external suppliers), and when you have to do this you cannot satisfy all preferences. Then, it is discriminatory to satisfy some preferences and disregard others because they are more common, or they have an established name ("vegetarianism" vs "not eating pineapples shipped by air freight"), or they are motivated by religious reasons. As long as the diet cannot be justified by a medical necessity, you are accommodating a personal preference, no matter the underlying reason; and it is discriminatory to favor some at the expense of others, and impossible to rate whether preferences are "valid" or "serious" (except by a doctor, for the medical cases).

It is probably a good idea to offer diverse food options to increase the odds that people find something that suits them, but as soon as you go out of your way to satisfy a specific observance, I think it has the potential to be discriminatory.

> Not providing foods guided by a person's religion is probably a violation of article 5 and is a violation of article 18 of the universal declaration of human rights.

If you interpret the "right to observance" as implying that the State, when detaining you, must accommodate your beliefs, then as I have argued it is practically impossible to do this in a non-discriminatory fashion.

(To give more background about my opinions: I am quite influenced by French laïcité https://en.wikipedia.org/wiki/La%C3%AFcit%C3%A9 whose current interpretation is often that, to avoid discriminating against any religious creed, as you cannot recognize them all, you should endorse none.)