cdn.discordapp.comは安全ですか？

This is the CDN (content delivery network) for the Discord app. It's not browseable or searchable, you can only see the content another user has directly linked you to, and direct links that work off-platform expire after several days. It can be misused in the same way as a file sharing site like Dropbox or Google Drive, but attachment size limits and the relatively short lifespan of any specific URL makes it difficult to use as a host for anything off-platform.

Watch out for deceptive content propagated in spam email which fraudulently claims to be from 'Shell' or 'Bank of America'. Watch out for spying web beacons concealed in spam email. These scams have are ongoing for several months, so I have no expectation that the site's administrators will put a stop to them.