bash -c "`echo echo hi`"
note that `echo echo hi` is fully read, and then (and only then) passed to bash. echo -c "`curl <your url>`"
The curl command isn't detectable as an evaluation because it's fully spliced into the string, then sent to bash. It's easy to imagine setting up a `curl <url> | sponge | bash` middleman, too.
It’s tantalizing to think it’s possible to make that site right now.