Martin,
The link you referenced is quite old and the Snap Creator plug-ins have since been moved to the NetApp toolchest and off of github.
A direct link is here, assuming you are signed into the support center: https://mysupport.netapp.com/tools/info/ECMLP2346441I.html?productID=62112&pcfContentID=ECMLP2346441
SnapCenter is taking over much of the capability of Snap Creator and the SnapManager products. Eventually you will want to move from Snap Creator over to SnapCenter.
SnapCenter has some of the same plug-ins as Snap Creator, with the ability to create your own plug-ins.
Here is a blog entry where the SnapCenter custom plug-in capability is being used to protect jfrog: https://newsroom.netapp.com/blogs/take-application-consistent-backup-of-jfrog-artifactory-with-snapcenter-2-0-on-ontap/
Hope this helps,
John