This is now a fact that Pages v14.5 cannot copy/paste into Pages v15.1.1, or the reverse. Your only workaround now is to use v15.1.1 and later as the sole Pages application on macOS Sequoia v15.6 or later, as the version 14 distribution has reached end of life.
Your older Pages documents that you could open in earlier versions of Pages should open just fine in Pages v15 or later where interdocument copy/paste will work as expected. I just opened a Pages document created by Pages v5.6.2 in El Capitan without issue in Pages v15.1.1.
Pages v15 and later are standalone from the Apple Creator Studio unless you deliberately subscribe them via the Pages application menu in-app purchase.
As a writer, do you want to know what version of Pages created the document, and all of the versions that edited same? Something like this?
