Last updated: September 17, 2021.
When you look under the hood of an embedded device, you may find out that it is running an ancient Linux kernel. Usually this will be one of the so called “longterm” or “LTS” kernels – a version that gets important bugfixes for a number of years, typically six. The latest such version is 5.10 (as of April 2021) released on December 14, 2020. The support duration for this one shown on the official kernel site is only two years. In February 2021 there was a discussion about the length of this support, with the options being two or six years. It is finally six years, and you can see the difference from the kernel releases table in April and September 2021 below.
The ordinary kernel versions get support only until the release of the next kernel version (for example, 5.11 will be supported until the release of 5.12 and you can check the support duration history at a dedicated Wikipedia page). Users are expected to move to the next one. However, on embedded devices, developers rarely change the kernel version, they usually support a single one during the whole lifetime of the device. The reason for this is the number of out-of-tree kernel changes they have. Out-of-tree kernel changes consist of all modifications done over the mainline version. Examples of such change include drivers not included in that specific kernel version (and added later), or received directly from the vendor, board-specific fixes and more. Linux kernel changes internal APIs between versions, so supporting out-of-tree changes might be complicated, as out-of-tree changes must be updated and tested for each kernel version.
For embedded developers who do not have out-of-tree kernel code, keeping up with the latest version might be a solution, as it is for application developers. The reason is simple. Contrary to internal APIs, one of the strong kernel development rules is avoiding breaking userspace (external APIs). Application developers should then see no regressions when upgrading from an older kernel version to a newer one.
Supporting a kernel version for 6 years means important effort, and makes sense only if there are devices using it. In addition, there is a need for people who will be test those kernels before a release. This is why the stable kernel maintainer asked for companies and project to declare if they want to use 5.10, and plan to support the testing. There have been some answers posted publicly showing engagement. It seems likely now that the support will be extended, but likely we need to wait a little bit more to know for sure. The decision has not been taken yet.