Hi, most of the sceneries will work in X-plane 11 as well as in X-plane 10.
About Aircraft, it's a different story.
If an aircraft is made for X-plane 10.20 or greater, it will generally be compatible with X-plane 11.
The best thing to do is probably to keep your copy of X-plane 10 if you have enough space, just for backup in case the aircraft you want is still not updated for X-plane 11. Then you should check on the developers websites, or by directly contacting them if they do not provide the information, or on forums like X-plane.org.
This forum here is only for X-plane default install.
Then it really depends : some developers provide free update for X-plane 11, some don't, and some are still working on the update.