Almost Illegal Strings, Mark II (Cops)
Python 2, 5 bytes (cracked by tsh)

bin()

Try it online!
Banned: all characters except alphanumeric characters
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789

So, you're allowed the above characters and nothing else (in addition to bin() itself).
(This idea isn't mine, I remember it from an earlier challenge, but I don't remember who deserves credit for it.)