TWRP 188.8.131.52 is out now for all currently supported devices.
What's new in 184.108.40.206:
- MTP fixes and improvements - you can now copy zips to the root of storage - thanks to _that
- MTP will now tell the host PC that storage is removed instead of disabling MTP completely
- MTP will now report the correct max file size based on the file system in use to the host PC (may fix transfer of large files)
- Update and improve fix permissions and make fixing contexts optional
- Update SuperSU in TWRP to 2.40 and update install process
- Make TWRP work properly on AArch64 (Nexus 9 is now built in true 64-bit binaries and libraries)
- Attempt to set correct permissions and contexts on all files placed in storage so backups will show in Android
- Fix kernel panic during MTP start on some devices
- Support unicode fonts on devices with True Type Font support
- Fix slider value not showing sometimes (vibration settings page)
- Toggle MTP off during adb sideload to set correct USB IDs
- Reduce library requirements for 5.0 L decrypt
- Other minor fixes and improvements
Head over to the devices page to find install instructions and download links.