Possible but difficult -- as is the cast with most things data pack. We obviously can't change the game's code, so just limiting the stack size isn't an option. What this would involve is checking, every tick (1/20 of a second, the refresh rate of minecraft servers) if a player has x food item by checking for EACH slot. That's 37 slots (4 rows and 9 columns with the offhand/shield slot = 4*9+1) that have to be checked every game tick if the player's inventory is full. The Count nbt tag for each slot with an item for each filled (non-empty) slot must be assigned to a scoreboard so we can check if it's over x stack size. If x item is over x stack size in x slot, we'd have to give the player that same item but in another slot, so /give (the easiest option) is out of the question. That just leaves directly assigning the item -- perhaps by putting the nbt data of the item into storage and then retrieving the storage with a modular `place_in_available_slot.mcfunction` function -- directly to the lowest available slot. This obviously wouldn't be perfect, and I'm sure this could leave, if not implemented very carefully, to bugs that either allow item duplication or delete items.

TL;DR without the technical mumbo jumbo: For each players' non-empty slots (slots with items in them):

• Store the item count of the slot (each slot) to a number
  
• Check if the number exceeds x amount (should be assigned to a scoreboard for in-game configuration)
  

If it does exceed x amount:

• Subtract current stack count (e.g. 17) to max stack size (e.g. 16) to get the amount of items we need to "refund" the player -- the amount of items that exceeded the stack size.
  
• Find the lowest non-empty slot and place that item into the player's inventory with the Count being the amount of items to be refunded.
• Profit

What this would look like in-game: You have 16 Steak in the first slot of your hotbar. You shift-click 3 Steak from your furnace into your hotbar. For 1/20th of a second (more if your ping is low or the server is lagging), you have 19 Steak in the first slot of your hotbar. The next 1/20th of a second, you have in your first slot 16 Steak and in your second slot 3 Steak. This short "visual glitch" (not actually a glitch, just the game doing what it's intended to do and the data pack fiddling with that) would happen every time a slot that has reached its artificial max slot count gets increased, i.e. when you shift-click or pick up an item into an already "full" slot.




LMK if you really need this pack. I can make it, but I only have so much time to spare. I recommend you learn how to make your own data packs. They're not TOOO hard. :^)