"Should" has never expressed a legal requirement. For years, the law schools in the US stressed that the word "Shall" is a contractual legal requirement to be fulfilled. The
The Supreme Court of the United States ruled that “shall” really means “may” – quite a surprise to attorneys who were taught in law school that “shall” means “must”. In fact, “must” is the only word that imposes a legal obligation that something is mandatory. Also, “must not” are the only words that say something is prohibited.
Here are some references that say to use the word “must” instead of “shall”:
Footnote: U.S. Supreme Court decision was in Gutierrez de Martinez v. Lamagno, that under certain contexts, shall could be construed as may. The decision does not imply that shall always means may, but rather that unless expressly defined, context determines whether shall is mandatory or precatory. As a result, "must" and "must not," without context, are the only words that impose a mandatory legal obligation or prohibition.
Interestingly, "Shall" continues to persist in contracts today, independent of this ruling.