With today’s release of ArcGIS Pro 2.4, you can now create an use network datasets with public transit schedule data! No separate downloads or installations are necessary. Learn more about the new functionality in the ArcGIS Pro documentation. A new tutorial can help get you started.
In light of this new functionality, the Add GTFS to a Network Dataset toolbox is now officially deprecated. No further enhancements or updates will be made to that tool. However, the tools in the Transit Analysis Tools.tbx toolbox, which was formerly part of the Add GTFS to a Network Dataset download, are not being deprecated. Instead, those have been moved to a separate downloadable toolbox. These tools have overhauled to work with network datasets created using either the old Add GTFS to a Network Dataset toolbox or the new functionality in ArcGIS Pro.
Feel free to discuss the new ArcGIS Pro functionality and post questions about it here in this GeoNet space. However, keep in mind that you can now call Esri Support for help with any functionality in core ArcGIS Pro, and that's often a better way to get help. For help with any downloadable toolboxes, you still need to post your questions here.