Today, cluster server parameters can be disabled on FCV downgrade via the min_fcv or feature_flag IDL tags. However, the IDL generator does not mandate that one of these tags be present for every cluster server parameter.

If a cluster parameter is created without either of these tags, then it will remain enabled even after FCV downgrade. This can create problems when the actual binaries are downgraded since the downgraded binary will exhibit different behavior than the FCV-downgraded server.

Mandating one of these tags will ensure that cluster parameters will always be disabled during FCV downgrade if they do not exist in the binary corresponding to that FCV.