Hi Joel,
Yes, I'm supposed to write a knowledge base/technical article about it...and got distracted about halfway through.
Short info is that all new transformation files--both horizontal and vertical--are being put into the ArcGIS Coordinate Systems Data setup. GEOCON v1.0 is there and at ArcGIS Desktop 10.6 / ArcGIS Pro 2.1, we also added NTv2 files for Canada, Switzerland, Spain, and the United Kingdom. On the vertical side, we added geoid files for Japan and New Zealand.
Also at these versions, we merged the ArcGIS Pro all users/per machine and the ArcGIS Desktop/Server CSD setups so you only need to install it once. If you're using ArcGIS Pro per-single-user, there's a separate CSD per-single-user setup. Single-user setup is usually done if the person installing the software is not installing as an administrator. In that case, the software gets installed to the user's account location (C:\Users\<user name>).
You don't have to install everything in the CSD setup. There are sections for
North America (GEOCON)
NTv2
Vertical World 1x1 (the 990 MB EGM2008 file)
Vertical (all other vertical files)
so you have partial control over what to install.
Melita