According to Idaho v Freeman, the old extension of the ratification deadline of the ERA was invalid:
Therefore, the Court is persuaded that the congressional act of
extending the time period for ratification was an improper exercise of
Congress' authority under article V. While Congress is not required to
set a time period in advance of the requisite number of states acting
to ratify, if it chooses to do so to remove uncertainty regarding the
question, it cannot thereafter remove that certainty by changing the
And in case that wasn't clear, the court says the following in its order:
The Court further declares that Congress' attempted extension of the time for the ratification of the twenty-seventh amendment was null and void.
The court also said:
the Court declares that a state has the power and right to rescind a prior ratification of a proposed constitutional amendment at any time prior to the unrescinded ratification by three-fourths of the states of the United States properly certified to the General Services Administration;
which would affect the count of states needed, as several have indeed voted to rescind.
This was a federal district court that made the ruling. The Supreme Court eventually dismissed the case as moot, since the purported extension had already expired by then. They also vacated the district court ruling. Nobody can say for sure how the Supreme Court would rule if Congress decided again to extend the deadline, but they might or might not find this case persuasive.