Is this the first such case since 9/11?
9/11 was different, that also required all planes to land, more or less, as quickly as possible and not continue to their destinations (unless that was timely, I suppose). This one is a departure ground stop, no more planes going up. Planes in the air haven't been ordered to land yet.
No.
"January 11, 2023. All U.S. departing flights were halted for several hours after a computer outage crashed the FAA's Notice to Air Missions (NOTAM) pilot notification system"
9/11, 2023 and today are the only three times.
no, there was one in 2023 when a pilot notification system went down