The main breaking changes added in 1.20.1 are that pretty much all default folder names have changed. For example, /data/namespace/functions has changed to be singular: /data/namespace/function. I have a full list of changes here:

• structures -> structure
• advancements -> advancement
• recipes -> recipe
• loot_tables -> loot_table
• predicates -> predicate
• item_modifiers -> item_modifier
• functions -> function
• tags/functions -> tags/function
• tags/items -> tags/item
• tags/blocks -> tags/block
• tags/entity_types -> tags/entity_type
• tags/fluids -> tags/fluid
• tags/game_events -> tags/game_event

If you're updating from pre-1.20.5, then there are even more changes: all item NBT has been replaced with item components. For example, instead ofgive @p pufferfish{display:{Name:'{"text":"Babel","color":"gold","italic":false}'},CustomModelData:123}it would begive @p pufferfish[custom_name='{"text":"Babel","color":"gold","italic":false}',custom_model_data=123]
Unfortunately the only way to update this is to do it manually, which can take a bit of time. Luckily, there exists a tool to convert old NBT to components: misode.github.io/nbt2components.

If you're updating from a version before that, there are going to be even more breaking changes which I cannot list all of them, but you can check the datapack technical changelogs at misode.github.io/changelog/?tags=breaking

I have the same problem! :/

https://minecraft.wiki/w/Data_pack There is everything that mojang has changed in datapacks in 1.21.