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Made a chart
>inb4
Kanto (OG), Unova and Kalos leagues have an specific order in-universe as seen in the badge checkers. Characters also mention the order of some trials/gyms in Alola/Galar too so the regions where you are free to go to any gym in any order (in the lore) are post-time skip Kanto, Johto, Hoenn, Sinnoh and Paldea.
Obviously the boxes with leaders/Elite Four/challengers represent when you have to beat them all but without a predefined order, hence the use of letters (A, B, C, etc)
