Well, the latest CME version tested & supported with UCCX 8.5 ( latest release as of yet ) is CME 8.6. If no further CME versions are mentioned in UCCX comp matrix literally means " not supported ". FYI, UCCX 9.0 onwards CME support will be discontinued & only CUCM will be supported, hence Cisco might not continue testing CME 8.8 & later with UCCX.
Pls rate the post if it helps.
GP.