This may not be definitive, but when talking about contact center deployments, if Cisco does not state support for a given PCCE/UCM combo, it's not supported. There are a lot of call flows that are validated during the solution testing that occurs. Typically PCCE does not perform testing with older versions of UCM....nor newer version. Unless the contact center BU specifically tested an older PCCE release with a newer UCM, you will not find it on the support matrix. Looking at the support matrix for PCCE 9.1, there is no support for any UCM other than 9.1.
If you are upgrading your UC solution to 10.5, then you will need to plan for a PCCE upgrade to get to a version that is supported with UCM 10.5.
Thanks,
Dan Keller
Technical Marketing Engineer