Only place I recall being mentioned is this FPN, but I don't think there is any code stating it is not allowed.
424.9 General. All fixed electric space-heating equipment shall be installed in an approved manner.
Permanently installed electric baseboard heaters equipped with factory-installed receptacle outlets, or outlets provided as a separate listed assembly, shall be permitted in lieu of a receptacle outlet(s) that is required by 210.50(B). Such receptacle outlets shall not be connected to the heater circuits.
FPN: Listed baseboard heaters include instructions that may not permit their installation below receptacle outlets