Consensus, so far as I know, has no numerical definition, but is thought of as being more than simple majority. One approaches the right to use the word consensus when 60% or more. 30/50 is 60%.
5
I understand the argument, but if the US Constitution changes its meaning automatically every time 60% of the states change their own laws, we're in big trouble. It would certainly seem contrary to the Constitutional provision that requires 2/3 of the states to approve any Amendment. We wouldn't need to follow the 2/3 requirement since every time the SCt found that 60% of the states agree, the Constitution would automatically change without an amendment.
Of course, as you point out the issue is not whether there is a "consensus" anyway, but whether it's "unusual". I don't see how they can say it's "unusual" if 19 of the states allow it.
I am sure that the death penalty opponents are now hoping they can get 31 states to abolish the death penalty, so they can rely on this decision to argue that the death penalty is "unusual" altogether, even for adults.