So we really want to drop WinJS if possible - it means shipping a few megabytes of extra resources with the C2 installer and this overhead gets bundled with every Win10 app, which increases file size, memory use and startup time. It also creates a bit of a distribution headache because technically we should keep our bundled WinJS updated with the official version, which we hadn't bothered to do previously (so there is the risk of bugs/unsupported versions as well).
I think pubCenter/Microsoft Ad SDK is the last thing relying on WinJS. I didn't find this while testing the changes in r228 since WinJS is not directly used by the pubCenter plugin, it's an indirect dependency. I've got in touch with Microsoft and they say the Microsoft Ad SDK is going to publish an update in the near future which drops the WinJS dependency, which should then fix the pubCenter plugin in C2 as it is. I think I would rather wait for this to happen rather than re-bundle WinJS, which adds overhead to every game. The Ad SDK update should happen before the next stable update (or if it's close we will delay the stable release for it).
There are two workarounds for the time being:
1) roll back to r227 stable. Note we have always advised that beta releases are just for testing, so you should not upgrade important projects without making backups and having a rollback plan in case there is a problem with the beta release. We cannot promise that problems only occuring on beta releases will be fixed any time sooner than by the next stable release (if indeed we decide it needs fixing). The hack-your-project-backwards trick only works if you don't use any features that were added in the intervening releases, which it looks like you did do, so it won't backport until you delete those changes. It looks like you started using the new "Set back button visible" action which was new in r228, which is why it won't backport to r227.
2) add WinJS back yourself. Start a new blank WinJS project in Visual Studio, then copy-paste the WinJS library stuff to your existing project including the referencing tags in index.html. That is basically what C2 does when it bundles WinJS, so adding it back yourself will get the Ad SDK working again.
I'll leave this report open as a reminder to others (and to me to test it works again before the next stable release after the MS Ad SDK updates).