Most C4C users are certainly aware that SAP has switched off HTML5 with the 1911 release and that it is - according to SAP - "technically impossible" to re-enable HTML5 from 1911 onwards.
I did have some doubts if a) that is really true (it's "only" a frontend) or if that was just to encourage customers to switch to Fiori (which they should - takes some getting used to, but I've come to like it better) and b) if it is a good idea to make it technically impossible to switch back as they will have certainly forgotten some features.
Turns out: They did and you can :-). To be fair, we tested Fiori thoroughly and SAP delivered all missing features until 1911, so there was no need for my customers to go back.
However, I've heard from other sources who were not as thorough in their testing and that SAP did help them out with one of their backend flags.
So if you find yourself in the same situation: Open an incident and SAP should be able to help you out.