Just for reference, on Legendaries:
the last time a list like this was datamined, it did include Pokémon like Legendaries and starters that weren't part of the regional dex and did not have dex entries, so the fact that they aren't on this list isn't just "because they're going to be in the game but not in the dex" (nor does a Pokémon being on this list guarantee that it has an actual regional dex number, in the case of something like the starters).
I can only assume the list that was datamined was just... not 100% current as of the announcement of 230+ Pokémon, rather than assuming that the list is final and looking for reasons that Pokémon they already planned wouldn't be on it.
(For what it's worth: even if this was just a rough draft, I assume they would have at least known about a major feature with every Legendary; I actually find it much harder to believe an all-the-Legendaries feature is part of the plan if there's no sign of it yet, compared to just... a random assortment of additional lines they hadn't selected at the time of this list that happen to increase the number to 230.)
If I had to
guess, this is probably all they had decided would be in the dex... at whatever time they made a new fork to focus on DLC separately from what they had to prepare for the Pokémon Day patch, but that could easily have been weeks or even months before the patch actually came out.
I'm sure
most of the DLC prep has taken place in its own fork as soon as they really got into developing it, whereas the Pokémon Day patch would have been intentionally based on an older version of the code that was closer to the original release.
Anyone working on something like this is going to make backups all the time, but more specifically, I'm pretty sure Game Freak uses something like Github that automatically records their commit history (see the Gen VII development repository leaks from a while ago, for instance), so they don't even need to decide when to make backups - they could roll back to whatever version they wanted at any point.
What this means is that they're probably not going to
start a relatively very small update like the Pokémon Day patch by taking a relatively current iteration of the code in the middle of DLC development and then manually scrubbing all of their progress so far, because it's much
easier (more practical / less likely to go horribly wrong) to roll back to an older version from before they really got into it, when there was much less progress on the DLC in the first place.
Whatever version they decided to base this patch on is just going to be the one they decided was the path of least resistance, which boils down to "it would take less effort and be less disruptive to scrub the tiny amount of DLC content that was already in this patch than it would be to copy over the content we actually want to release, like bugfixes and UI improvements, to an even older version with no DLC progress whatsoever."
Basically what I'm saying is:
if the rest of the Legendaries aren't on this list, what it realistically means is "there just were not plans to add back the rest of the old Legendaries, at least at the time this list was made, and there are significantly more plausible and more mundane reasons for it not to be 230+ already;"
the last remaining slots in the 230+ number could just be any random lines they chose to add back in between then and now, and there might not even be as many of them as people think (the press release that gave the 230+ number said it
included Pokémon that would be transfer-only, so it could reasonably include any number of the things we already knew about from HOME as well, just probably not stuff like Charizard/Cinderace/Greninja that was already out by the time the announcement was made).
But like.
Cutting down on legacy content bloat that they don't feel like they have anything interesting to do with / that only exists for the
sake of trying to cram in "every <x thing> ever" is, like, the entire point of cutting the National Dex. Why do people not think this is exactly the kind of pattern they're willing to break now?
They definitely
could choose to add some or all of them back later in development and I'm not saying it's impossible, especially since what we have is still an incomplete list and the Indigo Disk's release date is a long way off... but on the other hand:
This Gen introduced... I think 16 non-Legendary Paradoxes that are functionally Legendary, another 2 new Paradoxes that are restricted Legendaries, and 4 other new Legendaries already,
and then the DLC seems to be coming with at least 5 more new Legendaries,
and then HOME is already bringing back something like 20ish old minor Legendaries and another 10 options for restricted Legendaries when they do GS Cup.
They probably just said "that's plenty" and didn't want to cram in the other 38 things that were in SwSh under the same umbrella. P:
It's not like they could possibly keep doing this in every region... seriously, just imagine doing it again as early as next Gen: they're going to have, at a minimum, another 11 Legendaries from SV to consider the
next time they do this... and also all of the SwSh Legendaries and Enamorus are now legacy content as opposed to new content, so that's
another 14 more than the last time this actually happened.
Last Gen showed us that both starters and Mythical Pokémon could reasonably be on the chopping block for individual games - why are people still taking for granted that Legendaries can't ever be, to the point of "correcting" the actual datamined information to account for them instead of taking at face value that they just aren't in the datamine?