I added Minetweaker/crafttweaker support to my mod in 1.11. But now in 1.12 alot of methods are gone and all classes have been renamed at first why?, Is there any big reason or just because of old name -> new name? And also back in 1.11 I used minetweaker.MineTweakerAPI#registerClassRegistry(String) to regsiter everything... what has happend with this method and how do I fix this ?

1. The api changed, the old api was semi based on reloading, which is no longer a thing, so big changes had to be made.

you can use @ZenRegister to automatically register the class.

Take a look at existing examples :)
https://github.com/jaredlll08/ModTweaker/blob/1.12/src/main/java/com/blamejared/compat/actuallyaddition/AtomicReconstructor.java

SO @ZenRegister works alot like @mod or @JEIPlugin ?

yes