I don't know for sure but I would guess that they realized that a bit more arch to the back makes the braces pretty much redundant. Since the back is laminated anyway, it would be fairly easy and pretty cost effective to design it so that the arch is strong enough to work without the extra expense of braces and their installation.
If you think about it it makes sense. The back braces in most flat top guitars are there to support a thin back that started out flat and ended up arched because the braces force it to be arched. If the back is formed with a built in arch it really doesn't need the braces nearly as much. I've never seen a decent arch top guitar with back braces. They may exist but it would be the exception rather than the rule. The back just doesn't have the same stress on it as the top. Even with string tension pushing down on it, an archtop guitar has much less bracing built into the top than a flat top.
I was going to use the same example that Mark used: Guild Flat tops. They've been in constant service since the late 60's and the design is both stable & and accepted. Of course, vintage Guilds are just wonderful guitars..period.
I don't know if you knew this, but back in the '50s, Martin used to take a special guitar to trade shows. It was a D-28 but the sides & back were made of reinforced paper mache. They used it to show folks (this is Martin's claim, not mine) that the material & bracing of the sides & back have a smaller effect on sound than most folks thought. I learned this from an 'old timer' (today that's me!!) music store owner in 1970 who had been a Martin dealer since the early '50's. He said the guitar didn't sound any different than the real D-28 next to it!
Again, it's an OK thing.
because of the laminated arched back on the Taylor in question the Glue between the laminates is what is referred to as a Floating Brace so it is braced :: )
I have a Gibson Gospel Dreadnaught with an arched non-braced back in mahogany. I am guessing this design choice may save money on the production/bottom line?
FWIW: Searching ... The link below claims: "The Gospel guitar was designed to be a deep sounding vocal backup instrument." It also mentions Guild and Framus.