Mir 2.15 isn't the most exciting on the feature front but does roll-out MirAL 4.0 for this abstraction layer used in supporting Wayland. The MirAL 4.0 milestone drops a number of previously deprecated APIs.
Mir 2.15 does bump the AL, server, and Wayland ABIs for its libraries. The Mir documentation has also shifted to being online-based at readthedocs-hosted.com .
Mir 2.15 does improve its alt-tab behavior for window management with its MinimalWindowManager implementation. Details on that via this pull request along with a video of the improved behavior (screenshot from the video above).
More details on the Mir 2.15 changes via GitHub .