Bad news for everyone who has been waiting for the next Revit 2019 Extensions release. Autodesk recently published the following statement concerning the fate of Revit Extensions:
As of April 20, 2018, Autodesk will not deliver Revit® Extensions for Autodesk® Revit® 2019. Instead, customers using the timber and reinforcement modeling extensions can easily migrate to alternative solutions delivered by technology partners.
Revit 2018 is the last version that supports Extensions.
For more info check out an article at Autodesk blogs.