Theory: There's a possible relationship between the non-Galar legendaries obtainable in the base game of SWSH and new/upcoming anime episodes/movies.

[VetProf]

With the release of Pokemon Home, several non-Galar legendaries and mythicals that were previously found in the data of the base game of Sword and Shield can now be transferred to those games. Here's the full list of them from Serebii. The choice of legendaries and mythicals seems weird and kinda random, but I think there's a possible connection between some of these legendaries/mythicals and anime episodes/movies that were released/announced recently. I'll start by explaining the possible reasons Mew, Mewtwo and Celebi are available in the base game.

Mew is obtainable via the Pokeball Plus, which gives it a pretty valid reason to be available in Sword and Shield from the get-go. Mew is also central to Gou's goal in the anime and is closely related to Mewtwo, whose movie recently got a remake that was released in Japan last year and will be released on Netflix soon. An event Mewtwo was distributed as a pre-booking gift for the movie, too. Mew and Mewtwo are basically two peas in a pod, so if one of them is in the game, might as well throw in the other. As for Celebi, the shrine that appeared in the original 4ever movie can be seen in the trailer for the upcoming Coco movie, which greatly implies that Celebi will have some sort of role in the upcoming movie (in addition to the new mythical that will be revealed soon). If Celebi will appear in the upcoming movie, an event Celebi will probably be given away too as a promotional tie-in to the games (i.e. what they always do with movies). Anyway, my point is that all 3 of these Pokemon seem to have anime/movie related reasons to be in the game.

The next Pokemon on the list is the most suspicious and the main focus of my theory: Jirachi. Out of all the legendaries and mythicals in the base game of SWSH, Jirachi stands out the most. It's the only Gen 3 legendary/mythical in the base game, while Gen 1, Gen 5 and Gen 7 had multiple legendaries/mythicals in the base game. Celebi is also the only Gen 2 legendary/mythical in the base game, but if Celebi does appear in the Coco movie, then its presence in the base game makes sense since it will most likely be given away via an event. In addition, among the 6 pixie mythicals (mythicals with 100 in every base stat), only Mew, Celebi and Jirachi are in the base game; Manaphy, Shaymin and Victini are completely absent. As aforementioned, Mew and Celebi have plausible anime/movie related reasons to be in the base game, which really makes Jirachi the odd one out. The only way Jirachi's presence in the base game would make sense (according to my theory) would be if it's going to be available via an anime/movie-related event. In other words, if my theory is correct, Jirachi will most likely be featured in an upcoming movie, possibly the one after Coco.

This leaves... everything else. The Gen 5 dragons and the Swords of Justice might be hinting towards another Unova movie, maybe something similar to the Keldeo movie that we previously got. The Gen 7 box legendaries and mythicals might simply be here just because Gen 7 is still a relatively new generation, but then why aren't the Tapus here? Also, Marshadow and Zeraora might be here due to their relatively recent appearances in I Choose You and The Power of Us, but this doesn't explain the absence of Ho-Oh and Lugia. Melmetal's probably here just to mark its first appearance in a proper main-line game. They even gave it a G-max form, so they probably have something planned for it.

Honestly, there's a lot of holes in my half-baked theory. This theory mainly came from my attempts to rationalize the seemingly-random choice of legendaries and mythicals (especially Jirachi) in the base game of SWSH, which probably led to me overthinking and looking too much into things. Regardless, on the off-chance that I'm right, I'm just gonna leave this post here and pray that the immense number of Gen 5 and Gen 7 legendaries in the base game doesn't mean we're getting another Hoopa movie.

So, what are your thoughts on my midnight rambling?

EDIT: Fixed a typo

 

[Silktree]

By next year every legendary will be in SwSh due to the DLC (second part). I'll grant you Celebi, but that's about it.

 

[VetProf]

By next year every legendary will be in SwSh due to the DLC (second part). I'll grant you Celebi, but that's about it.

They might've prioritized putting certain legendaries and mythicals in the base game in preparation for future movie-related events, but yeah, the second DLC will probably be out by the time the Coco movie is released. Still, there has to be a reason for this seemingly random assortment of legendaries in the base game, which is what bugs me the most.

 

[Silktree]

The weirdest part is that there seems to be a staggered distribution of an Original Color Magearna to those who have completed the National Pokedex. Magearna is currently unobtainable in SwSh.

Ah, the many joys of Dexit.

 

[Nicolas721]

Most of the Pokemon of the Pokemon have their own reasons for being in the code, though:
-Bulbasaur, Squirtle and Mewtwo are very popular and even non-fans would be wondering where are they, so they were included to prevent fan outrage. The first two are already available in Home. I don't think they are there for the anime.
-Celebi is indeed there because of the movie.
-Jirachi is there for distracting us from Celebi, but more likely, it could be distributed in Tanabata.
-The Unova trios are there for Go promotion maybe.
-The Alola starters, legends & mythicals are there because they have been in the least amount of games.

The anime could cover them, yes, because it's a multi-series region where any place and any Pokemon could be covered the next episode, but I doubt that kind of choices would have been taken in approx. mid-2019 when these Pokemon were coded and the Alola League was starting.

Another hole is that the DLC is coming in less than 5 months away and could include its own set of Transfer-only Pokemon, and also all legendaries will be obtainable by the end of the year. Transfer-only Pokemon will be just that, Transfer gifts that will be "legal" by the end of the year.