In the medium term we would like to remove the ability to specify more than one distro. Historically it was useful in using static hosts and dynamic hosts in related pools, but that's not been a reality of our deployment recently.
At the same time the logic to support allowing tasks to appear in more than one task queue makes task dispatching and scheduling much more complicated than it might be.
In the future we can add a layer inbetween distros, which describe host configurations, and the distros which represent queues, using some sort of pooling or grouping system, which will provide more flexibility as it will be an application wide configuration, and not specific to task definitions.
This deprecation and warning is the first step in this process.