They absolutely have issued public warnings before…
The announcement includes…
First, utilizing a group buff to progress in solo Greater Rifts is not intended and will be considered cheating. Players found using this mechanic may have their accounts suspended or closed, based on severity of abuse as well as previous account history.
We ask that you do not make use of this exploit in the meantime while we are working to deploy a patch.
From a later post in the same thread…
Now that we’ve identified it, we’re asking you do not abuse this problem while we work to address it—for the greater health of the game.