It's FOSS - https://itsfoss.com/linux-lts-kernel-six-years/
Brief: Linux Kernel Long Term Support (LTS) releases will now be supported for six years instead of two. This will enable Android devices to survive four OS upgrades.
Linux kernel has two kinds of releases: Long Term Support (LTS) releases and the regular releases.
A regular Linux kernel release arrives at an interval of 8-10 weeks. This means that devices running regular Linux kernel must be updated every 10 weeks.
This is where LTS Kernel release comes to save the trouble of upgrading devices every 10 weeks. An LTS release comes with 2 years of support and provides security and bug