For maintenance reasons, the interactive shell for the GRASS C++ Plugin, as well as the embedded QTermWidget, have been removed from future releases.
It is expected that the GRASS Processing Plugin will be given priority development and maintenance support going forward, and providing an interactive GRASS shell is not a critical functionality for QGIS itself but rather something that users may manage externally.
Further details on the specifics of this feature cleanup can be found on the feature Pull Request.
This feature was developed by Nyall Dawson