There are no Apple employees participating in these strictly fellow user-supported, public communities. We can't answer or even speculate about "why" questions on behalf of internal Apple product team decisions.
If you review some of the other preceding posts in the Pages community, there have already been several discussions about the copy/paste discrepancy between Pages v14.5 .vs. Pages v15.1.1. Apple never expected users to co-exist both application versions, since Pages v14.5 will not receive future updates. Apple even throws a dialog suggesting Pages v14.5 removal.
Pages v15 and later does not impose any mandatory subscription to Apple Creator Studio. There are features within Pages v15 that can be unlocked with a Studio subscription, but again, that is optional. I have opened older Pages documents without issue in Pages v15 and documents created with it have opened in considerably older versions of Pages.