Aware of what? Government says a file is illegal? Sounds like a censorship regime to me.
Not if the key is provided to the platform operators to confirm the contents. Otherwise yes anyone could claim any encrypted file contains illicit material and people would game that system.
For what it's worth the key may not need to be manually shared to the provider as referrers often leak where people learned about the file and that source location may also contain the key or password. All it takes is one person using a web interface or addon that leaks such information. Some addons break the referrer-policy header and many website operators don't even set the header [1] in the first place. Example header testing [2]. Please test the sites you visit and kindly ask the website operators to address any missing headers.
# nginx example
referrer-policy "strict-origin" always;
Often is the case but I would still suggest setting the referrer policy should the file be enticing enough for people to register an account assuming forum ranks and further actions are not part of the picture.
