We'd expected that backporting
SERVER-48048 immediately would obviate the need to understand the previous yml syntax, but that doesn't seem to be the case; it looks like we use previous release versions in multiversion tests rather than the tips of other branches.
We should add back in the logic from earlier CRs of
SERVER-48048 to handle this. Right now everything in etc/backports_required_for_multiversion_tests.yml is being unconditionally excluded.