Does the FBI or any other law-enforcement office follow up on these backdoors? Is this considered a crime, or even conspiracy to commit a crime, or is it only the act of using the backdoor that's a crime?

I can also see that it's still up in NPM without any warning of any kind: - https://www.npmjs.com/package/mathmain

But the Github repo for the package and the author are down: - https://github.com/allendev12 - https://github.com/allendev12/mathmain