Time-series collections are incompatible with earlier versions of MongoDB and must be dropped before downgrading. Creation of time-series collections requires a fully upgraded system that successfully ran the setFeatureCompatibilityVersion: "5.0" command.
Attempted downgrade using setFeatureCompatibilityVersion: "4.4" in presence of time-series collections results in an error.
Starting an older mongod with datafiles from a new mongod that supports time-series collections, the regular FCV mechanism provides a descriptive error message.