What is the most secure encryption algorithm that keeps the entropy the same or even lowers it?
This seems pretty simple if your 'isEncrypted' distinguisher looks for a cipher text entropy rate in excess of 7 bits byte. But it's not exactly an encryption. Just hex encode it.

All good cipher text is computationally indistinguishable from perfectly pseudo random.  Take the most securely encrypted cipher text from any decent algorithm.  It will be 8 bits/byte if occupying all the bits within a byte.  Then hex encode it to ASCII base 16. You now need two bytes to encode any single original byte of cipher text. Ergo the entropy rate has been halved to only 4 bits/byte and you still maintain maximum security. Your message would of course get correspondingly longer.  

In extremis, go to UTF-32 binary encoding which could just be a series of two really funky characters.   Perhaps two Egyptian Hieroglyphs of your choice, each occupying 4 bytes. Mean entropy rate = 0.25 bits/byte.