Pokémon GO was down on June 1 from 11 a.m. to 6 p.m. PDT. Read on below to learn more:
Pokémon GO will be down on Monday, June 1, 2020, from 11:00 a.m. to 6:00 p.m. PDT (GMT −7)
Pokémon GO will be experiencing global downtime for seven hours on Monday, June 1, 2020. From around 11:00 a.m. to 6:00 p.m. PDT (GMT −7), the game will be undergoing server maintenance and will be inaccessible to all Trainers. We apologize in advance for any inconvenience that this might cause.
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—The Pokémon GO team
Source: Official Pokémon GO blog