Well... I have successfully ported projects that were built using JUCE 3 with its polymorphic plug-in format (hence including the stand-alone app) to JUCE 4.3.0 fairly easily.
The best part of it is that they still happily build polymorphically and including the stand-alone app as well (we haven't done any effort to extend to AUv3 though).
I guess the lack of a stand-alone target is more a limitation of Projucer (and possibly its need to support AUv3) rather than of the JUCE library itself.
@otristan if you feel like experimenting, you could download a very old JUCE version (mine was 3.1.1 or 3.2.0 if I remember well), build the Introjucer and bootstrap a project with it. Then, fast forward the JUCE source to the current version and cross your fingers...