Mainstream end of support for Windows 10 21H2 LTSC is 2027-01-12. But the process to switch is not particularly easy nor is it free. You need a Windows 10 LTSC license.
We can expect that the next LTSC *after* Windows 10 Enterprise LTSC 2021 will be built on Windows 11. And it will also keep the 5 year support. However, that's expected in 3 years or so ...
If you are looking for an older version of Windows, check out the official list here. Lastly, these keys should only be used for temporary activation. Ultimately, you will have to buy a genuine ...