"Seila Law" is a law firm, not a law. They were a party in a recent SCOTUS decision, Seila Law LLC v. Consumer Financial Protection Bureau, 591 U.S ___, which involved the Consumer Financial Protection Bureau created by Congress, under the Dodd-Frank Wall Street Reform and Consumer Protection Act. The court held that the structure of that law (regarding appoinntment and firing of the director) is unconstitutional:
The CFPB’s leadership by a single individual removable only for
inefficiency, neglect, or malfeasance violates the separation of
powers.
More specifically,
Article II vests the entire “executive Power” in the President
alone, but the Constitution presumes that lesser executive officers
will assist the President in discharging his duties. The President’s
executive power generally includes the power to supervise —
and, if necessary, remove — those who exercise the President’s
authority on his behalf. The President’s removal power has
long been confirmed by history and precedent.
The law in question addresses a single position, director of CFPB, not all government agencies. "Agency" is defined in 18 USC 6 as
includes any department, independent establishment, commission,
administration, authority, board or bureau of the United States or any
corporation in which the United States has a proprietary interest,
unless the context shows that such term was intended to be used in a
more limited sense.
"Department" is then defined as
one of the executive departments enumerated in section 1 of Title 5,
unless the context shows that such term was intended to describe the
executive, legislative, or judicial branches of the government.
POTUS firing power is about the executive branch, not the judicial or legislative branches, so it depends on what you mean by "agency". You can infer possible outcomes based on this ruling, but what they literally said is that particular law is unconstitutional.
