Introjucer from Sound Radix's JUCE branch (with has audio plugin side-chain functionality) has an additional Config option for plugins - "Plugin Accepts Side-Chain" - see screenshot:
This option is not available in the Projucer.
Additionally, the initial plugin boilerplate code which this branch's Introjucer creates is compatible the branch and its API changes (channels are also indexed by bus).
If I understand correctly, I cannot use a Projucer that has these changes, as Projucer's source is not available and so I cannot merge our changes to it..
Of course, we could just use a preprocessor definition instead of a Jucer checkbox, but we incorporated the changes to Introjucer because we wanted to make the features we added easy for you to incorporate into main Juce easily and cleanly, and this option should probably be a checkbox just like "Plugin Midi Input" etc..
Having side-chain added to Juce would be great and would also partially solve this problem for us, and of course much more - having what we need incorporated in main Juce would mean a lot to us - more testing for the code and less maintenance work for us to do. For example the merge with JUCE's templated audio buffer changes, which I performed today, included non-trivial merging in the VST3 wrapper - that's time and additional testing (which we didn't perform yet) that we would had very much liked to save.. btw we do get some assistance and pull requests from other developers using our branch but we'll have more if it was in Roli's..
But even if main Juce did have side-chain - I recall fixing many different issues with Introjucer with regard to SDK paths and OS compatibility - problems and fixes I of-course reported here and have subsequently been fixed in the Jucers, but going forward this would mean that I will not be able to fix the Projucer when future problems arise..
I wonder if Projucer's features (which I didn't even get to try as my trial didn't seem to start yet) could be factored out of the open-source part of Jucer, or at least if it could have the side-chain features incorporated..