You've found all the right resources. The Platform Support Guide has sections in there describing what you are asking.

We typically do not test older Pega/PRPC versions on newer platforms (especially when they did not exist at the time of said release).

 

With that said, some combinations may work even if they were never officially tested/supported. Your results may vary depending on the exact scenario and product/version you are attempting.

Section 6 is probably the most relevant to your question here.

". Pega Platform and newer Platform Components: It is Pegasystems’ policy not to backwards-test new versions of Platform Components with older releases of Pega Platform. Support for new platform products is provided through newer versions of Pega Platform software. If you choose to upgrade Platform Components to the latest releases, but want to stay on older releases of Pega Platform, you may enter support requests for any problems encountered. However, if the root cause of these problems stems from the fact that the older versions of PRPC were not designed to handle functionality in the newer Components, Pegasystems may request that you upgrade your PRPC version to a Pega Platform release that directly supports that newer Component, as listed in this Guide."