The detailed list of changes for 1.12 can be found here:
According to the Twitter Feed of md_5, the lead developer of Spigot, they aim to release a working 1.12 build sometime on Thursday (tomorrow!).
We will update immediately to see if any plugins are negatively affected and in order to proceed with our plans as quickly as possible.
As usual, please report any bugs you may find after the update goes live!