If you jailbroke your device with Pangu 8.0-8.1, this untether has already been automatically installed for you.
Updating this package requires a reboot; please note that rebooting can surface problems unrelated to the untether or Cydia Substrate. If you've set a passcode/TouchID, rebooting may cause your device to freeze: please disable passcodes/TouchID until further notice (sorry :/). Having incompatible tweaks installed can also cause problems upon rebooting.
Version 0.2 of this package fixes an issue that caused Cydia Substrate to not fully load into many processes immediately after rebooting. After installing this untether update, you no longer need to respring after rebooting to make Cydia Substrate work.
This update also solves the Safari and iMessage bugs that some users reported. (For developers: the issue with RWX pages was also fixed.)
As this update makes Substrate work more consistently, installing this update exposed bugs in Substrate 0.9.5013 for many users. It is important to also use the latest versions of Substrate (at least version 0.9.5014) with this new untether.