from the dissent (starting on page 57):

The effect of the
Court’s order, followed by the Trump administration’s re-
peal of the rule, was that the Clean Power Plan never went
into effect. The ensuing years, though, proved the Plan’s
moderation. Market forces alone caused the power industry
to meet the Plan’s nationwide emissions target—through
exactly the kinds of generation shifting the Plan contem-
plated. See 84 Fed. Reg. 32561–32562 (2019); Brief for
United States 47. So by the time yet another President took
office, the Plan had become, as a practical matter, obsolete.
For that reason, the Biden administration announced that,
instead of putting the Plan into effect, it would commence a
new rulemaking. Yet this Court determined to pronounce
on the legality of the old rule anyway. The Court may be
right that doing so does not violate Article III mootness
rules (which are notoriously strict). See ante, at 14–16. But
the Court’s docket is discretionary, and because no one is
now subject to the Clean Power Plan’s terms, there was no
reason to reach out to decide this case. The Court today
issues what is really an advisory opinion on the proper
scope of the new rule EPA is considering. That new rule
will be subject anyway to immediate, pre-enforcement judi-
cial review. But this Court could not wait—even to see
what the new rule says—to constrain EPA’s efforts to ad-
dress climate change.
The limits the majority now puts on EPA’s authority fly
in the face of the statute Congress wrote. The majority says
it is simply “not plausible” that Congress enabled EPA to
regulate power plants’ emissions through generation shift-
ing. Ante, at 31. But that is just what Congress did when
it broadly authorized EPA in Section 111 to select the “best
system of emission reduction” for power plants.
§7411(a)(1). The “best system” full stop—no ifs, ands, or
buts of any kind relevant here. The parties do not dispute
that generation shifting is indeed the “best system”—the
most effective and efficient way to reduce power plants’ car-
bon dioxide emissions. And no other provision in the Clean
Air Act suggests that Congress meant to foreclose EPA from
selecting that system; to the contrary, the Plan’s regulatory
approach fits hand-in-glove with the rest of the statute.
The majority’s decision rests on one claim alone: that gen-
eration shifting is just too new and too big a deal for Con-
gress to have authorized it in Section 111’s general terms.
But that is wrong. A key reason Congress makes broad del-
egations like Section 111 is so an agency can respond, ap-
propriately and commensurately, to new and big problems.
Congress knows what it doesn’t and can’t know when it
drafts a statute; and Congress therefore gives an expert
agency the power to address issues—even significant
ones—as and when they arise. That is what Congress did
in enacting Section 111. The majority today overrides that
legislative choice. In so doing, it deprives EPA of the power
needed—and the power granted—to curb the emission of
greenhouse gases.

tl;dr: the majority chose to rule on a plan that never went into effect and will never go into effect, in practice taking on an advisory role rather than waiting for an actual concrete plan to rule on. Beyond that, Congress intentionally chose the broad language of "best system of emissions reductions" but the majority asserts that congress should have instead referred to generation shifting by name despite the technology not existing at the time of drafting