Correct, there is no plan to make Indigo Server changes that would cause Indigo Touch 1.7 to become incompatible. That might happen some day, but not in the near future.
We generally provide backwards compatibility very far back. For example, I don't know of many (any?) modern Mac OS apps that still support all the way back to OS X 10.5. We do this because we recognize many of our users are running their home servers on much older Macs. It is tough though because users also want the new functionality. As an example, we currently are facing the issue of users wanting to use python 2.6 or higher for plugins, yet Mac OS X 10.5 has python 2.5 installed. Supporting both python versions is not only difficult from a development perspective, but also confusing for end users because then some plugins won't be compatible on all Indigo installs.
Balancing these trade-offs is part of what we have to do. We have to recognize that it won't be possible to meet everyone's needs – tradeoffs have to be made weighing adding support for new functionality / OS services versus supporting legacy hardware / OS versions. In the case of the next major release of Indigo Touch, we have decided requiring iOS 7 or higher is in the best interest of a majority of our users and Indigo's direction and future (as well as our sanity on the dev/QA side
).