In an unusual sighting in 2020, Justice Thomas wrote a pretty good judgement for the whole court, explaining that waiver or notions of estoppel could be applied bya court to disable a ground for refusing enforcement of an arbitration agreement under Article II(3) of the New York Convention: https://www.supremecourt.gov/opinions/19pdf/18-1048_8ok0.pdf. This is all about the disabling of the formal validity rules in Articles II(2), something that has been going on in various firms since at least the 1980s.
Essentially, the court is following the minimalist view of Article II, namely that it sets out the lowest levels of acceptable obstruction to the enforcement of an arbitration agreement. Article II does not stop national courts enforcing arbitration agreements that do not meet the requirements of both parties’ signature or an exchange of letters or telegrammes. In this respect, the Court took the same view as the English Court in Zambia Steel & Building Supplies Ltd. v. James Clark & Eaton Ltd,  2 Lloyd’s Rep. 225.