Hardknott will not become an LTS version, and will be maintained for 7 months, per https://wiki.yoctoproject.org/wiki/Stable_Release_and_LTS (all of it worth reading).

The current plan is that Kirkstone will be the next LTS release (there's an LTS release every two years).

My recommendation is to start the project using current master (rather than something already released), periodically rebase on a later master, and branch off the stable/product branches in sync with upstream LTS releases. Rebasing from one yocto release to another yocto release is a lot more painful than rolling master + stable LTS model.

Alex