This applies to mongod/s version 8.0.x.

Currently, a cluster running 8.0.x binaries in FCV 7.x will not reject attempts to run setClusterParameter with a "fleCompactionOptions" object containing fields that do not appear in the FLECompactionOptions IDL schema. Note that IDL parsing permits unknown fields because the IDL definition in v8.0 onwards is non-strict. This presents a problem when v7.x binaries are then introduced in the cluster, as v7.x binaries have a strict definition of FLECompactionOptions, and so will fail upon seeing unknown or incompatible fields.

Note that since SERVER-95111, known fields that are in the v8.0 schema but are not in the v7.0 schema are already forbidden from being set whilst the server is in FCV 7.x.