I have no clear idea. It might have changed its business model and didn't work out as planned, or there could have been legal or copyright problems that affected it.
I'm not 100% certain. You might want to look at its privacy policy, the kind of content it offers, and whether it's been flagged by security software. That could give you a better idea of its safety.
To use Comics Code.com well, you need to register an account if required. After that, browse through the available features and categories to find what you need. Pay attention to the instructions and guidelines provided on the site for optimal usage.