>>31183166They're categorized the same way as minor legendaries in the game's coding, and there's no reason to assume the Pokédex identifies Pokémon as legendary on a case-by-case basis rather than according to said categorization. It could be the case that minor legendaries are no longer considered legendary at all, which would make sense considering mythicals are now treated as a subgroup of legendaries while minor legendaries are a separate group.
Surely if Game Freak was still using the same classification scheme, there would be a single category for all special Pokémon, which would be divided into one group for Type: Null and company and a one group for legendaries, and that group would be divided into one group for minor legendaries, one for major legendaries, and one for mythicals. The way they've grouped the Pokémon seems to indicate either that minor legendaries are no longer legendary, or they still are, and Type: Null, Silvally, the Tapus, and Ultra Beasts are legendary by association (and the Pokédex just doesn't call them that for whatever reason, but it wouldn't so so for older minor legendaries either). It's impossible to know which is the case until an official source confirms it, but it must be one or the other.
It seems the only argument against this claim is "b-but they don't FEEL legendary!", which is a baseless argument for several reasons. For starters, I'll give you Type: Null and the UBs, but if you think the Tapus don't "feel" just as legendary (if not moreso) than the birds or beasts, you're delusional. They're very clearly the same kind of group as the minor legendary trios, and actually have lore behind them. And secondly, Phione breaks every single established rule for a mythical Pokémon and is still officially recognized as such by the overwhelming majority of official sources. The bottom line is: a legendary is whatever Game Freak says it is, and the game clearly says they're just as legendary (or not) as Articuno or Regirock.