I imagine Cisco will call it out in the A2Q process if it isn't documented as being acceptable. You could also try raising a case and/or an exception with TAC to see if they can tell you why it isn't generally supported, but I know PCCE is limited in some ways in that manner.
I did, in fact open a case, and was given the same information that I had previously looked up. In any case, they are saying that they will be supporting 6.7 and 7.0 ESXi. At the same time, I was able to convince my customer to upgrade their ESXi environment ahead of our upgrade to 12.6. Crisis Averted thus far!