In light of the rather significant changes that Pokemon Go has undergone in the face of the spread of the Coronavirus, Niantic Support has confirmed that "The Shadowy Threat Grows", March's Giovanni research to allow players to capture Shadow Entei, has been extended into April in order to give players more chances to catch the Fire Type Shadow Legendary Pokemon.
Trainers, we are extending the end date for this month's Team GO Rocket Special Research, "The Shadowy Threat Grows," until the end of April, to allow for more time to finish the tasks and encounter Shadow Entei.
— Niantic Support (@NianticHelp) March 26, 2020
Fortunately, this Special Research line can be completed solo, so it's entirely possible to capture Shadow Entei while practicing social distancing. In the mean time, it seems that Shadow Suicune will just have to wait a little longer for its virtually inevitable turn.