Which Switch games are the only way to obtain certain Pokemon?

Question

I'm making a living National Dex to hopefully one day substitute in a living Shinydex, and I was thinking about which Pokemon I'd have to ask for on the GTS (in my case the BDSP exclusives) and which I can catch in the games I have.
Any non-timed encounter or gift counts.
If it is a gift, please say how you obtain/activate said gift.

Pokemon like Melmetal (before ZA Mega Dimension) should be mentioned that they cannot be found in a Switch game, but do say where they can be gotten (e.g GO, HOME gift, previous game)

Comments

Edited for a little more clarity, even if you now have to reread the title a few times to get it

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Cant melmetal be found in let’s go games?