Now that the game has been out for a while, I wanted to hear your thoughts on this. (PLOT SPOILERS)
PLA seems to be heavily based on the Meiji Restoration, a period of Japanese industrialization that lasted from 1868 to 1912. If RBY is considered the equivalent of 1996, this places this game around about 128-84 years before every other Pokemon game. Spiritomb is also said the be atleast a few hundred years according to Vessa, and Spiritomb is 500 years old as of modern day Pokemon games, so this all lines up.
The biggest thing I am unsure about, and why I made this thread, is which timeline this is in, the Mega timeline, or the non Mega timeline. XY and ORAS established two different continuities, the one with Mega evolution seen in Gen 6 and 7, and the one which doesn't which mostly consists of Gen 1-5 games. SwSh is unconfirmed on which timeline its part of but I'm going to guess its part of the Mega timeline. If you have any evidence either way I'd love to see it.
Legends Arceus is arguably even more ambiguous as to which one it's part of, as the only Megas that could possibly exist at this time were Mega Rayquaza and possibly Mega Lucario.
There is an ancestor to Wally, Beni, who uses a Gallade... alongside a Gardevoir, but has ORAS Wally's hair.
The paintings in the Diamond and Pearl clan settlements seem to depict a Alder ancestor... and ancestors of specifically the original RSE designs for Maxie and Archie.
The shirts the protagonists are wearing at the start of the game are actually from XY and SwSh respectively it seems.
Unless I'm missing something, there just isn't enough conclusive evidence to prove either way, maybe when Hisuian Snow comes out next week, or even a potential 1.2 update releases, we might learn more.
Hoooo, now we're talking! I'm always down for discussing Pokémon timeline stuff.
I agree with your headcanon here, but I also want to point out some things that I used for my own. As I see it, PLA takes place more or less 100 years before the events of D/P/Pt.
First of all, LGPE seems to have inaugurated a new timeline that puts the original Rocket incident from R/B/Y closer to 2018 than to 1996. If the timeline placements from the other two timelines are still correct, then the Team Galactic incident, which happened concurrently with or shortly after the second Rocket incident (G/S/C/HG/SS), would happen three years later, in 2021.
Now, 100 years earlier would be 1921, which in Japanese history would be in the Taisho period. However, that's precisely why it's fitting. The style of the more "modern" buildings seen in Jubilife Village, such as the Galaxy Team HQ, were more common during the Taisho period than during Meiji. Then, there's also Laventon's handheld camera seen in the end credits - a device that only became more common around the 1920s. Unless he picked it up from a time-space distortion, which is highly unlikely, it's a dead giveaway that this game is supposed to take place around the early 1920s. Then again, we have had someone build what is essentially a nuke/death ray three millennia before the events of X/Y, so perhaps technological advancements don't necessarily correspond with the real world's.
As for your question of whether this is the Mega timeline or not, I think that, with the question of the multiverse out there, it's irrelevant. From the moment Zinnia hinted at the existence of alternate universes, the games became less like a coherent unified timeline and more like a collection of events that may or may not have happened across timelines - with what all the remakes, third versions and reimaginings. So yeah, what I'm doing here is trying to pinpoint an accurate time frame while acknowledging I might be overthinking all of this.