Or, more specifically: the egg moves pages only show data for the most recent game the Pokemon appeared in. In Pikachu's Gen 8 egg move page, there are clearly only Pokemon from BD/SP (even when Sw/Sh is selected), and the list only includes moves according to data for BD/SP. (For example: Pikachu's egg move page says that it can get Thunder Punch as an egg move, but this is only true in BD/SP, and not Sw/Sh. The equivalent table on Bulbapedia has separate areas for Sw/Sh and BD/SP in recognition of that fact, as well as other egg moves Pikachu only has in BD/SP.)
I tested this myself in Sw/Sh to be sure; Lopunny indeed cannot pass Thunder Punch onto Pichu (and therefore Pikachu/Raichu).
In fact, Bulbapedia says Raichu and Ampharos are the only Pokemon (besides Pikachu) that can pass Thunder Punch to Pikachu in BD/SP. PokemonDB includes the Chimchar, Buneary, and Smeargle lines (and so does Serebii). Which site has it wrong? (I'll test it in BD/SP later, it requires chain breeding and I don't feel like doing that right now.)
On a related note, Pichu's egg move page has no tables despite the fact Pichu can hatch from an egg (if you breed using Pikachu or Raichu). It does link to Pikachu's page, but I still think the way it's laid out is confusing (especially when the main Pichu dex does list egg moves for Pichu, so people will click through to a blank page expecting matchup data). Shouldn't it simply look up Pikachu's egg moves instead?
