The test matrix covers every runtime version the project claims to support
A support claim you do not test is a guess. Dropping the oldest supported version from the matrix is how a patch release breaks half the installed base.
Observable check
the matrix entries in the CI configuration cover every maintained runtime release line included by the support range declared in the package manifest.
Where this comes from
No upstream source. This is oss-kit's own position, and the argument for it is below.
No specification ties a declared support range to a test matrix entry. Observed: the two live in different files, the package manifest and the CI configuration, and nothing on either side fails when they disagree, so a support claim decays silently until a patch release breaks the oldest line still in the range. Retire this rule if package managers begin rejecting a manifest whose support range the project does not test.