To be clear, all downgrades are forbidden. Sometimes a downgrade will appear to work (possibly silently losing data) and sometimes they'll fail as reported here, but downgrading to any earlier version is unsupported. This is even true for patch releases like trying to downgrade from 7.1.1 to 7.1.0.
This is not completely true. You can upgrade directly from any 6.y version to any 7.x version (for x ≥ 1) via a full cluster restart. If you want to perform a rolling upgrade you can only do this from 6.8.y to 7.x (for x ≥ 1).