Please be careful here: This only means that there is technical compatibility, the old stuff will work technically and can be loaded into the new sim and this does not mean that the old addons also work correctly. For example, you can use old aircrafts but the new sim will have a new flightmodel and the old aircrafts will not behave correctly with a changed flightmodel if they are not updated by their developers. This was the same situation with XP10 and XP11, old aircrafts could still be used back then but their flight behaviour changed drastically and they did not fly by their specs anymore.
That's maybe the case, curious for the result on pure for XP11.5x configured planes will not be reasonable compatible. A lot of the addons concerns scenery and that's my biggest issue with current AFS4 on lost money(+150$) that I invested in AFS2 addons made by 3rd party developpers.