Your Oculus Quest should receive an automatic system update these days (version 16). This update is known to remove a large random subset of applications from “Unknown Sources”. So in case VR Sketch disappears (or anything else):
first, try to reboot your Quest (hold down the button on the right of the headset for a few seconds).
if that doesn’t help, reinstalling it from scratch probably fixes the problem. It’s anyway a good idea to do that if you didn’t get the latest version (VR Sketch 11.0 at the moment)
Sorry about that, there is nothing we can do about it ourselves. Comments around the Internet describe it as a known problem in the system update 16.