I don't see the point in a 28 gauge combination. Maybe others don't either? So a reason they don't exist?

I think some of the multi barreled things four or five barrels have a sub 12 gauge barrel.

Lots of combinations have 16 gauge barrels.

Probably the sub 12 g use is to keep the combination lighter and trimmer in cross section with the extra barrel(s).

Never seen a 28 gauge shotgun myself.

Why not go for a .410 shotgun combination?