IE8 / XP
I'm with you on this. I have a customer that is currently transitioning to IE8. That's right - IE8 is new for them.
The reason is that they run XP internally and must run IE as they have internals apps that demand it (which is lame, but common). XP can only go as far as IE8. Upgrading to Windows 7 or beyond is a much bigger deal.
I think this situation is not unusual for large corporations or government bodies. As a developer I'd love to ditch support for old browsers, but it's just not realistic.
It seems odd to me to branch the project for a 12% reduction in code size due to removal of legacy features. In my view with the current market share of IE7 and IE8 that means jQuery 2 is unlikely to be of use in a real-world setting for some time.