Yes, in VS2019 all top-level plugin menus are moved into the new Extensions menu. This is done by Visual Studio automatically in an effort to make the top level horizontal menu manageably small. There are no "guidelines" per se, at this time.
Historically, GhostDoc's menu is located under the top level Tools and that's where all users know to find it. VS is not moving it since it is not the top level item.
We are considering the new Extensions menu but we would only do that after an extensive user survey and if the vast majority of GhostDoc users is favorable to the change.