There is no reason to duplicate that work upstream and then nobody uses it anyways.
And by the way, LF founs the Civil infrastructure project which mantains LTS kernels for 10 years.[1]
[1] https://wiki.linuxfoundation.org/civilinfrastructureplatform...
Debian definitely uses the upstream LTS for their base for their kernel on stable. They do add their own patches, but I don't think it's fair to say that they duplicate work here.
There is no reason to duplicate that work upstream and then nobody uses it anyways.
And by the way, LF founs the Civil infrastructure project which mantains LTS kernels for 10 years.[1]
[1] https://wiki.linuxfoundation.org/civilinfrastructureplatform...