Currently, querying the config servers still uses configOpTime() for the readConcern afterOpTime. However, if there is an initialized value (ie. not kUninitialized) for the VectorClock's ConfigTime component (and FCV is initialised and indicates kVersion451 or later), then that should instead be used (with afterClusterTime, not afterOpTime). (For upgrade/downgrade purposes, if these conditions are not met, then the server should fall back to the existing behaviour of configOpTime() for afterOpTime.)

configOpTime() is also used for the readPreference minOpTime (here and here). It's not clear what should be done about these (since configOpTime() is to eventually be replaced by ConfigTime), but SERVER-46499 should help guide this.