This is weird, I have messages syncing in iCloud (beta only feature). It had been working for me with the older beta builds, in fact, it worked as intended. With 10.13.14 it has been buggy, out of sync and not all inclusive. I have it enabled on my MBP and iPhone X. I just noticed that messages are taking up 10.8GB of my space per the list but not reflected in my 93.5GB of 200GB storage. I wonder when it is released if this function when enable won't count toward your storage? Probably more likely is that it is beta so they aren't counting it 'yet'. Although I could have sworn I had a green bar labeled messages using my storage in prior betas?