Trying to load a previously stack size of 64 for unstackable items gives this:

[21:36:03] [Server thread/ERROR]: Tried to load invalid item: 'Item stack with stack size of 64 was larger than maximum: 1'

Instead of limiting the count to the max_stack, it just does not load the item at all.

Steps to reproduce:

Use an older version of Minecraft to generate an item stack with more items than max_stack_size.

E.g. 64 diamond swords. Make sure this item stack is inside a shulker box or bundle.

In 24w09a, load the world. Your item stack will be gone.

I have not confirmed this in scenarios such as enderchest, or directly in inventory, saved hotbar, chests or other inventories.