Hi Erik,
Merging decisions are up to the Chrome Release team (or the Security Team for security fixes). You can always request a merge (by adding a Merge-Request-xyz label to the bug), and argue why you think it should be done, and see what they decide. Make sure to tag the bug with the right OS label, otherwise it won't get looked at.
ChromeOS does care about older branches, they even backport security patches to their LTS releases that don't get refreshes on other OSs any more.
(Also, I think there's a typo in the first line of your mail: when Chrome 130 goes to stable, then V8 *12.9* usually doesn't get updated any more. V8 *13.0* does definitely get security fixes throughout its lifetime as a stable release.)
Does that answer your question?
Cheers,
Jakob