FWIW, the whole notion of having a LTS release seems primarily being pushed/supported and worked on by a single core developer, and it seems rather unlikely that a single contributor can maintain a "long-term-support" binary if others don't step up to also get involved.
Given FlightGear's constraints as a project (manpower, expertise, number of open bugs/merge requests, people with commit access, people able to triage bugs etc) it's actually very ambitious, not to say surprising, to even toy with the idea of releasing a LTS version at all.