Ubuntu Wiki

The kernel feature freeze is a deadline for any new-feature related kernel features. As feature freeze is generally too early to freeze the kernel functionality (as around that time usually the final upstream release of the kernel is not yet available), the kernel feature freeze marks the point after which we can consider the kernels for a release feature-complete and expect only bugfix changes to land afterwards.

The timing is set in such a way so that the Beta images have a full-featured kernel in place for testing.