NDK r26 will be an LTS release. Note that this release is also labeled as beta 2 for compatibility with Android Studio.
The main highlight is that this release includes an updated LLVM and libc++. We're reworked things so our libc++ comes from the rest of our LLVM distribution, so all future LLVM updates will also include libc++ updates.
r26 will be the first release that does not support KitKat (API levels 19 and 20).
See the changelog for changes and announcements.