oh, we actually have the following text:
This threshold is subject to change by the opam repo maintainers. The threshold is based on the oldest ocaml compiler version available in the “maintained” distributions. It determines the minimum compiler version used in tests for the opam-repository CI.
Where “maintained” has a footnote: The versions of ocaml shipped by linux distributions are accessible on repology. Both homebrew and macports are usually very fast to move to the latest available. Which LTS are still maintained isn’t explicit, but it can be found at endoflife dot software (e.g. https://endoflife.software/operating-systems/linux/ubuntu).