The plugin does not install inside the maya2019 folder anymore, it now installs in a similar structure to how Arnold does.
On Windows, it will go to C:\Allegorithmic\SubstanceMaya\2019\...
On MacOS, /Applications/Allegorithmic/SubstanceMaya/2019/...
And on Linux, /opt/Allegorithmic/SubstanceMaya/2019/...
The 1.4.0 version that shipped with Maya 2019 also installs in the same way.
We did this for a few reasons. It's much easier to manage being outside of Maya's install directory and the plugin can now exist or be moved anywhere, as long as the .mod file is updated to point to the new location. This is something studios have been wanting for a while.