I would suggest the following change to the Help Center page on "What topics to ask", under the last bullet in "Questions you should avoid":
- Support for Ubuntu releases past their "End of Standard Support" or "End of Life" dates (whichever is earlier) — unless the question is asking how to upgrade to a supported release.
Is changed to:
- Support for Ubuntu releases past their support period (5 years after first release for LTS and 9 months after first release for non-LTS) — unless the question is asking how to upgrade to a supported release
This is not taking into account a different support timespan for Ubuntu Core though.
Alternate suggestion if Ubuntu Core support is 10 years (as it currently appears to be - pending clarification):
- Support for Ubuntu releases past their support period (10 years after first release for Core, 5 years after first release for LTS and 9 months after first release for non-LTS) — unless the question is asking how to upgrade to a supported release
This would in my opinion make the paragraph more precise and easier to understand. And there would be no doubt, that there's up to 3 time frames you should know for a release to expire here - 10 years (Core), 5 years (LTS) and 9 months (non-LTS).
The way it's phrased right now, if I was a new reviewer, and hadn't read the meta post, I would assume the on-topic period is 5 years for standard Ubuntu and 3 years for official flavors. In addition, the current guidelines do not mention Ubuntu Core at all.