Subject phrase: {That such a (reversal) of precedent would be countenanced}
predicate: was
predicate complement: {itself unprecedented in the court {, a bastion of traditionalism} }
{a bastion of traditionalism} is a noun phrase in apposition to the court.
The court was such a bastion of traditionalism that for it to not object to a reversal of precedent was unprecedented.
Why would be?
The court is so traditional that its countenancing of a reversal of precedent is contrary to expectation. The modal would be stresses the "remoteness" from fact or expectation.