So since SV came out, I've had a list of how many regional pokedexes each pokemon has been in, since I thought it'd be interesting to see which pokemon appear a lot, which ones get screwed over, etc. Note that, to avoid repetition, I only included went with the largest pokedex from each region/game, including DLC dexes with the base game dex: so only Platinum and not DP, only USUM and not SM, Paldea and Kitakami and Blueberry Academy as one pokedex, etc. But Hisui and Legends ZA are still included as they're different games with much different pokedexes rather than being an upgraded version.
Anyway:
After gen 5, only four pokemon lines had appeared in every pokedex: Zubat, Psyduck, Magnemite, and Onix. Onix wasn't in the Alolan Pokedex, bringing it down to just Zubat, Psyduck, and Magnemite, and it remained this way for a very long time. Then Zubat was never in Scarlet and Violet, bringing it down to just Psyduck and Magnemite.
What does any of this have to do with Legends ZA or this thread, you're probably wondering? Well, this game marks a historic moment.
Psyduck and Magnemite aren't in this game, not even the DLC
This means that, from this point onward, there are no pokemon to make every game's (expanded) pokedex. While there was a brief moment after SWSH's launch where this was also the case, as all three of those lines weren't in the base game, they still appeared in the DLC, making this temporary and making it so the final dex used for that game still includes them. This time however, unless the leaked returning pokemon list is wrong, they will never make it into this game, and therefore there will never be a pokemon featured in every game's pokedex ever again. Kinda surprised it took this long tbh.
Interestingly enough, there is one Johto pokemon that has never missed a pokedex since its creation; Heracross. Therefore, Heracross is actually tied with multiple kanto lines for the pokemon with the highest amount of pokedexes it is in.
(Also, for anyone wondering about Pikachu, it surprisingly was never in Unova, not even in BW2)