I don’t know if shouldn’t’ve is grammatically correct but I hear it a lot so it seems like fair play. Same for other contractions that I never see in text, possibly because they’re wrong. Because’ve. He’d’ve.

Also like I’ma which can’t possibly be ok, but “I am going to” is for suckers.

Would’ve: fine. Would have: fine. Would of: me go mental! Why do people do this?! Argh!

I will accept “would ve” before “would of”