Silicon Labs took the decision to deprecate its proprietary implementation of Thread and instead be a major contributor to the more popular OpenThread implementation.
The announcement was made as part of the release notes for Silicon Labs Thread (SL-Thread) SDK v18.104.22.168 which can be found here:
The notice indicates that the inclusion of the SL-Thread implementation will be removed from the Simplicity Studio release scheduled in December 2019.
The notice re-iterates that Silicon Labs has a continued focus on its Thread strategy and is committed to continuing as a leading-edge Thread supplier. Support for Thread will continue within the Silicon Labs software and hardware tools but the stack availability itself will be distributed through the commonly accessible OpenThread GitHub repository.
More information on the OpenThread implementation can be found at: https://openthread.io/platforms/efr32