Fedora just happened to get there first...but other distributors are making the same choice to ship the newer gnome-session even with this regression in place. Maybe because other pieces of gnome are starting to make use of features in the new gnome-session design and simply keeping an older gnome-session with have other impacts on the overall integrated desktop.
These comments really speak to the competing expectations projected onto the people working at the distribution level.
Very few distributors promise a feature regression free experience across the distribution release boundary. Why? Because very few of the upstream projects themselves promise a regression free experience across their release boundaries, even minor releases.
But as users that seem to be what we expect from distributors. We try a distribution, we like how the pieces work together and stick with it, and we want it to work that way..forever...regardless of what design decisions are being made in the upstream projects to support backwards compatibility or legacy behaviour. I think its a logic fallacy to expect distributors to do a better job at being upstream developers than the upstream developers themselves.
Fedora certainly does not come anywhere close to even hinting at an implied promise of a feature regression free experience across the Fedora release boundary. Fedora self-describes itself as leading edge, and each release is focused on integrating the current upstream project offerings.