I have noticed that the 28 gauge is usually part of a brace of skeet guns. Yes, I've heard of the 28 guage, but I have never seen 28 guage shells for sale in any of the sporting goods store I once frequented.
I suspect the lower recoil is preferred along with it might be a tad more sporting because of the lessor shot than a 12 or 20 gauge gun.
Best deal on 28 ga Winchester AA is at Wally World.
Pheasant are larger and tougher so you use a 20 or 12 gauge for those, and for Turkey and Duck or Goose it's usually a 12 or 10 gauge with Magnum shells.
There is a great deal of science and physics to it because the shot sizes change as well.
With all that said, a 28 gauge is usually the first choice on a skeet field for women because of it's low recoil, and for those shooters who are so expert that they prefer a greater challenge. My 5'1'' 103 lb wife shoots my Benelli 12 gauge just fine, but we're getting her a smaller gun this year so she can shoot more regularly. We're going to try her with a 20 gauge because the shells typically cost less, but if it's still too much for her we'll go with a 28.