The Pokemon Timeline is confusing and convoluted, but it can be figured out. This is what I discovered.
For starters, the version-exclusive Pokemon determine which games are connected to which timelines. Growlithe, for example, are all a part of the Red/Gold timelines. While Vulpix is part of the Blue/Green/Silver timelines. This applies to legendaries you can get outside of their native region as well. Third game expansions count as their own separate timelines, like Yellow/Crystal/Emerald/Platinum.
Secondly, there are different multiverses and this was confirmed in OR/AS. There's at least four major multiverses: the Classic timeline, on the Game Boy games, the Advanced timeline, which are generations 2 through 5, the Mega timeline, which are generations 6 up to Ultra Sun and Ultra Moon. And the current Switch timeline, which are of the Nintendo Switch games from Let's Go Pikachu and Eevee onward (the Switch games all being separate from the previous timelines, having a completely revamped TM numerical system, accessible PC boxes anywhere, no HMs, mandatory Exp. Share forced onto players with no way to turn it off, limited RNG for wild battles in tall grass, and for the most part, no Mega Evolutions.)
All of these changes - typically for the worse - heavily imply all of the Nintendo Switch games are not in the same timeline as the Mega universe, especially the new TM classification order and the new battle mechanics. The version-exclusive Pokemon mostly remain consistent throughout these timelines, letting you know what games directly connect to their sequels.
- CLASSIC TIMELINE A: JP Red/Gold
- CLASSIC TIMELINE B: JP Green/Silver
- CLASSIC TIMELINE C: Red/Gold
- CLASSIC TIMELINE D: Blue/Silver
- CLASSIC TIMELINE E: JP Blue/Crystal
- CLASSIC TIMELINE F: Yellow/Crystal
.
- ADVANCED TIMELINE A: FireRed/Sapphire/HeartGold/Pearl/White/White 2
- ADVANCED TIMELINE B: LeafGreen/Ruby/SoulSilver/Diamond/Black/Black 2
- ADVANCED TIMELINE C: FireRed/Emerald/HeartGold/Platinum/White/White 2
- ADVANCED TIMELINE D: LeafGreen/Emerald/SoulSilver/Platinum/Black/Black 2
.
- MEGA TIMELINE A: Omega Ruby/Y/Moon
- MEGA TIMELINE B: Alpha Sapphire/X/Sun
- MEGA TIMELINE C: Omega Ruby/Y/Ultra Moon
- MEGA TIMELINE D: Alpha Sapphire/X/Ultra Sun
.
- SWITCH TIMELINE A: Legends Arceus/Let's Go Pikachu/Brilliant Diamond/Sword/Scarlet
- SWITCH TIMELINE B: Legends Arceus/Let's Go Eevee/Shining Pearl/Shield/Violet
The anime specials bridges some of these past events in new timelines that didn't get remake games, like Pokemon Origins showing you some of Kanto's events in the Mega timeline (Red's Charizard can Mega Evolve), Pokemon Generations showing off Johto, Sinnoh, and other regions during the Mega Timeline, and Pokemon Evolutions showing off older regions during the Switch Timeline.
Lastly, there was one reference (I forget which) that said Legends Arceus was specifically 150 years in the past from the gen 8 present (150 because of the original Pokedex number.) Most of the games take place in the equivalent present-year as the real world. So that would place Legends Arceus around the year 1869, which is also the real world period of the Meiji era of Hokkaido development in Japan, which was from 1868 to 1912.
there doesn't seem to be anything here