If you are doing that, shame on you, that's not how stable and long-term-support works at all. If you backport, you backport on a best-effort basis, not on a must-have basis.
There is 0 sense in supporting old versions when you just backport all the new stuff into them anyway.
>You, in your GitHub project, probably don’t have the time or resources to manage more than one release line of development.
That is certainly true. Over the whole essay I don't find much reason not to have a LTS schedule.
I don't have to backport all the stuff to older releases. My own policy is that any previous release will receive security patches as long as I can be made to care (usually about 1 year and in one case I support a 3 year old release of a project with security patches)
It is not that much effort to patch security stuff. It doesn't take you away from your project for days, it costs you a few hours and some users of your project will be happier for it. That's all I want.