It is important to note that this "old version" is effectively unsupported. The developer has moved on to v7+ branches that use a different API approach. However, due to the nature of YouTube’s backend, as long as the ancient Android TV API endpoints remain active, version 6.17 724 should continue to work.
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Smart YouTube TV is the older, legacy project. SmartTube is the newer, actively developed successor by the same developer. SmartTube offers more modern features like SponsorBlock integration, up to 8K and HDR support, and a more polished interface, but the 6.17.724 version is often preferred for its stability on older, low-resource devices.