I did a preliminary test of spigot 1.12 and found the following plugins broken:
- Dynmap (our nice 3D map). This is fairly widely used and was last updated this February, I would assume a fix will be available soon.
- Modifyworld which makes sure that Guests cannot break anything, prevents certain blocks in certain worlds and is a part of the whole Permissions setup on the server. This one is really critical. This one has not been updated since September 2013. There is a fork that has been updated recently, but only the source code. I need the Jar file for that.