As was previously reported, players who have updated Pokemon Go to the latest version on iOS devices is causing numerous issues, including problems with Rocket encounters.
In response, Niantic has temporarily disabled Rocket encounters for all players.
Trainers, while we investigate errors associated with Team GO Rocket encounters, they will be temporarily unavailable. We'll update here with more information. We apologize for any inconvenience.— Niantic Support (@NianticHelp) April 10, 2021
Given the fact that Shadow Pokemon hunting is currently considered to be one of the more important end-game activities in Pokemon Go, this move is a fairly large hit to the community. We currently have no word as to when this issue will change or any degree of compensation that may be offered to players once it's fixed.