I wanted to chime in on this as well, as I’m trying to undergo certification and the ‘Core’ Activities have also been removed from my UI Path development environment. This has brought me to a dead stop unfortunately as I can’t access any of the Core System activities.
I’ve done a few hours of trouble shooting, including reinstallation, scrubbing all associated data, and following what I could find online for a work around and documentation. The funny thing is, everything used to work just fine, and I had a project that had ‘Input Dialog’ and other activities that functioned, even though I couldn’t see them in the activities list. I opened up UIPath today morning, and the activities were just flatly gone.
I really need to get acquainted with UIPath and finish the certification, as I believe it’s a technically superior tool in terms of Application design compared to other RPA products, and I want to be able to fully understand and recommend it. Will be watching this thread with high hopes.
 I also checked for UiPath.Core.Activities with Filter Activities unchecked.