My launcher has notified me there's an update available but I haven't clicked it. All of my mods have been working fine. There's only one with a flag on it, though the mod page says "1.20.x". I'll remove that one and test it. Edit: Clarification: My game is 1.20.3. I have not updated to 1.20.4. All of my mods have 1.20.x in their tags.
Edit: Deleted the mod, opened the save. Same result. Immediate crash upon load in, same message.