They just shipped support for Mountain Lion, which was released one year ago. I hope there is not as big a delay for Mavericks. I understand that it's a volunteer project, but a kickstarter could surely help them muster the $100 for a Mac developer subscription and access to the developer previews of 10.9.
10.9 is being actively tested internally and we've already released two hacked together preview versions for it. Doing everything we can to be on time this time for real.
You're underestimating the level of effort here. It's not about lacking the $100 (they've received more than that in donations), it's that Apple makes significant changes to the way their applications are built and run and they don't communicate the changes very well, if at all.
In addition, as the apps are linked and compiled with different settings (clang, no ppc, no 32-bit, etc.) then plugins that are dependent on these APIs have to find their own way.
ML was a huge change from Lion in many ways. Mavericks has some issues that an individual can resolve on their own (mostly) but it's a moving target as it is not released yet.
But I do encourage you to donate. Integration with your native mail app should be worth $50-100+.
