It is and it has been since 1787, but there’s no functional difference between a law not being enforced and the thing the law’s about being legal.
(Art. I, § 9, cl. 8): “[N]o Person holding any Office of Profit or Trust under [the United States], shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State.”
(Art. II, § 1, cl. 7): “The President shall, at stated Times, receive for his Services, a Compensation which shall neither be encreased nor diminished during the Period for which he shall have been elected, and he shall not receive within that Period any other Emolument from the United States, or any of them.”
(Art. I, § 6, cl. 2): “No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office.”