MAJOR OVERHAUL:

There's honestly just been way too many changes to actualy list them all here so I'll go over it generally.

Version change: 1.20.1 -> 1.21.1
Mod Loader change: Forge -> NeoForge
Multiple mods removed:
Ai improvements
AntiPortals
Apotheosis Ascended
Better compatability checker
Better ping display
Celestials shaders
Chat Heads
Death counter
Drippy loading screens
GPU memory fix
INventory HUD+
Item Borders
Longer Chat history
Loot Journal
Lootbeams
MUlti Overworld
Player shops
.......
Multiple mods have been replaced:
EMBEDIUM → SODIUM
Elenai Dodge & Feathers → Combat roll
It takes a pillage → It Takes a Pillage Continuation
Wild Lands/Bountiful → [Let's Do] WilderNature
Connectible Chains → Reconnectible Chains
Nekos enchanted books → Beautiful Enchanted Books
Oculus → iris Check wat nieuwe shaders
AUTO LEVELING → Dynamic Difficulty
PASSIVE SKILLTREE → Pufferfish's Skills
.......
Multiple mods have been added:
(I lost that list, oopsie)

Alot of balancing adjustments have been made aswell, mobs are abit less lethal and level a whole lot slower with distance (except for Y level)
Mob spawn points has been changed alot. You won't find every mob in every biome, some have been restricted to certain custom "biome groups" while other have been restricted to certain structures

I think that's about it. I've done a bunch of testing for a few days and I think I have run into the biggest ones but please let me know if you find anything else!