Is it possible to 'derate' a plane as a safety measure?
The answer is yes, if there is an operational procedural restriction limitation that effectively mitigates the risk, without adding its own risks, and is something that is operationally feasible, pending a design change.

Examples would be an AD that:

Imposes a speed, altitude, weight or temperature limitation.

Imposes additional reserve fuel requirements (this was done for CRJ200s that had a problem with the flaps getting stuck while extended following a missed approach).

Limits operation in icing conditions (although for most airline operations this effectively grounds you, because "legal" icing conditions exist in any cloud above the freezing level).

All sorts of other system restrictions.