I've thought about this, and I'm in favor of it. It doesn't hurt to be specific, and there's a fanbase for everything. Without this tag, it could be a lot of tags, such as: cbt, guro, body modification, amputee and so on (no guarantee of completeness of my list) and yet it's not exactly that.

Castration removes the testes or ovaries, while sterilization only prevents reproduction without necessarily removing the organs. it's not male or female exclusive.

Castration is a word that applies specifically to males and their testes. If you wish to include ovaries in it the tag should be "sterilization" instead.

because "castration" and "nullo" are two completely different levels of extremes. You castrate animals on a daily basis. "nullo" is just horror

Body modification is a benign tag while castration can a more extreme tag. This is a specific type of 'body modification' and nullo is an even a more specific type of this and 'body modification'. Why are you okay with one but not the other?

I just remembered that someone suggested create tag nullo +9, which is much more needed

I'm personally fine with just "body modification", I don't see the need for a more specific tag