Yeah, but that call just kinda seems, I donno, silly. Sorry.I'm not sure I see the relevance of the question, since purify food and drink needs to be cast on Supply and create or destroy water doesn't create Supply.
So the "Supply" concept was to basically simplify the food/water consumption requirements and make it easier to then modify them with abilities and such.
The book says rations are basically a day's worth of food and water, and 1 unit of rations provides 1 supply (p329).
And T&T goes into more detail on this on page 55
"Mundane consumable items like food and water are simplified into a single item called Supply. When an adventurer gains access to food and water, they can add Supply to their inventory. Basic rations (dry food and water) usually costs 5 silver for 1 Supply, but finer foods may cost more.
• 1 Supply consists of enough combined food and water to sustain a Small- or Medium sized creature for a day."
And the optional Desperate Supplies rules on the same page lay out that 1 supply is basically 1 pound of food and 1 gallon of water.
But this doesn't mean that 1 supply = exactly 1 days food and water. For example fishing traps provide 1d4 supply in fish. That's food, not water, but it still counts as supply. Meanwhile medium mounts require 1 supply a day. But it's a rough system no an exact one. You can't feed a riding horse on fish.
So supply works for the purpose of simplifying food and water requirements.
So specifying that Purify food and drink won't work on food or water but only on supply is kinda...dumb. If it's stripping all toxins out of food and water then it's making nonpotable water potable. The change doesn't really work unless the desire is to make create water and purify food and drink both far less useful. Because again, all a "supply" is, is about a days worth of food and water for a person.
Though page 363 of the AG also says that create or destroy water is supposed to create supply. Though the statement that supply (eg food and water) cannot be stored in extradimensional spaces also makes no sense.
Now a change that would make more sense, but would still weaken the spell for no real reason, would be to make Create or Destroy Water a duration spell. Once the time runs out the 10 gallons of water disappears. Meaning that you get a lot of temporary water or just 1 gallon of good permanent water.
Or just let Create or Destroy Water create 1 gallon of fresh water, or 10 gallons of non-potable water which can then be made pure by burning a Purify Food and Drink to make a number of gallons potable equal to the caster's proficiency bonus.