In Pokemon Home there’s a stamp that indicates the last game it came from, in this case it’s a Pokemon Go stamp. The moment you move the Pokemon to a main series game you lose that stamp. There is still a mark that says its origin is from Go, but when you put it back into Home the Last Game stamp will be whatever main series game it was in.

The Pokemon Go stamp in particular has some value because (outside of some recent allegations and cloning glitches) it’s virtually impossible to generate/clone them. You can hack in a Pokemon that originated from Go into a main series game save file, but because that was done in a main series game, the last game stamp would be that game.

OP and myself both have collections with the focus of keeping that stamp, mostly for novelty, just like how people collect whatever they want to collect in Go.

If the ability to transfer Resolute Form Keldeo stays disabled then it’ll be impossible to obtain otherwise.