I don't know if it's a "trend" but with lighting going to LED's instead of other methods, then yes channel letters and cans are getting shallower. We install a lot for the nationals and reverse channel letters are often no more than 2 1/2" deep, sometimes down to an inch.
It's a bit easier to fabricate a 2" return instead of a 5" return, meaning less material, so then it weighs less, which costs less to ship, etc. Shallow cabinets have been popular for decades with companies that mass produce cabinet signs, at least when they have pan faces.
