The Jetson TK1 Developer Kit is approaching EOL. Final orders from distributors have been received, and NVIDIA will continue shipping Jetson TK1 Developer Kits through April, at which point those orders will be fulfilled. JetPack 3.1 is the final JetPack to support Jetson TK1 Developer Kit.
Note that NVIDIA Tegra K1 SoC will remain available through January 2024.
L4T sustaining releases for Tegra K1 will continue to be made as deemed necessary. (L4T 21.6 was the most recent release.)