Thanks to @SimonKubica for keeping the conversation up and thanks to @JasonPhan for joining in! 
This is essentially what I expected and I completely understand that rebuilding this API (or a similar one) would take considerable time. I also understand that it of course doesn’t have the highest priority as it wasn’t used much.
This one I don’t quite understand though, since the API kind of has reached EOL already, right? I appreciate the openness, but I guess the decision you have to make in June is probably rather whether you want to build a successor, right? (Edit: And if you do - allow me to change theme values at runtime please! 
)
Until then, could we maybe have the broken/EOL/zombie theming module removed from the docs? We often evaluate whether new app ideas or server->cloud migrations are technically possible. (This is actually how I encountered that this module is broken.) Having this broken module in the docs is actively hindering cloud migrations because it leads to vendors wasting time on trying to use it, like in my case. Removing it from the docs hopefully shouldn’t be too much effort and can hopefully happen before June. 
Cheers,
Sven