Pokémon: Detective Pikachu appears to be set about 20 years after Ash's time, according to the voiceover explanation when Pikachu looks at the hologram in the research center (timestamp 1:02:54). It's reasonable to think that the events in "the Kanto region" were Ash's adventures.
Dr Ann Laurent: It [Mewtwo] escaped nearly 20 years ago from the Kanto region.
On the other hand, director Rob Letterman doesn't seem very comfortable talking about it, as the "live action" Pokémon universe's connections with the "main" Pokémon universe aren't precisely defined yet.
Screenrant: I liked how this movie exists in the world of Pokémon but doesn’t overstate references. But one that really stood out was Kanto, in reference to Mewtwo. Does that imply Team Rocket exists in this world, and that they’re fully functioning?
Rob Letterman: I can’t answer that question without getting into enormous trouble. You know your Pokémon well, but you’re putting me on the spot. The fact that Mewtwo is in means that Mewtwo storyline is accurate and all that stuff exists.
Screenrant: On a similar note, would you be able to comment on whether Ash or Red exists in this world?
Rob Letterman: We haven’t thought through where we land in the overall timeline of the Pokémon universe, but we’re connected in one big universe. We didn’t work out where in time we are, because this is just the first entrée into exploring the world of Pokémon in live action.
Rob Letterman Interview: Detective Pikachu, Alex Leadbeater for Screenrant, May 15, 2019