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I'm not sure where this lore tidbit goes, but I found this in Heliolisk's entry:
A now-vanished desert culture treasured these Pokémon. Appropriately, when Heliolisk came to the Galar region, treasure came with them.
I'm interpreting this as an allusion to Britain looting ancient tombs, though...
For the regional dexes though, they're pretty good about it usually--remakes use the original dex entries, but only XY ported entries over for its regional dex. SM and USUM both had new entries (for SM and for USUM). Most of the repeats are from Pokémon that can only be obtained through transfer.
I've been going through the Pokedex entries, and while I'm only about 100 through, it seems that there aren't any new ones even for the returning Pokémon (excluding Pokémon that recieved Galar forms)? That's pretty dissapointing if so; even in previous gens, new dex entries for the regional dex...
Let's put "my uncle works at Nintendo" statements on the backburner for now and think about possibilities. One avenue moving forward for Pokémon is to do exactly what they said they would: releasing future games with a subset of existing Pokémon and moves while reserving the "total" dex for...