That's the thing though: it's a retcon (or, if you prefer the full name,"retroactive continuity"), in which changes are made to a setting or characters to better suit the audience experience, and retcons have been a thing for at least as long as fiction itself. (Other examples include Superman getting the ability to fly without explanation, the back of the house in The Simpsons changing from a graveyard to the Springfield Nuclear Power Plant, and Classic Sonic the Hedgehog being 14 years old when he defeated Dr. Eggman for the first time in Sonic 1 but was 10 years old when that happened in Sonic Generations.)
Yes, it's a retcon, but that doesn't make it any more acceptable. Retcons should always be a last case scenario. It should be a priority to keep the world building already established in-tact.
I don't see evolution methods as front-and-center, as all that NPCs ever talk about on the subject are that Pokémon evolution exists and the different ways you could do it. The only Pokémon-specific cases I can think of, which would be part of the lore and not just gameplay mechanics, are that Haunter evolves by trade (NPCs in Kanto trading Haunters and one of them being surprised that it evolved; Gaspar in Sinnoh holding an Everstone), that Eevee evolves via a Water Stone, Thunder Stone, or Fire Stone (NPCs in Kanto and Johto telling you how to get Vaporeon, Jolteon, or Flareon), and that Poipole evolves in a way other than friendship and leveling up (Ultra Recon Squad in Alola attempted them and weren't successful). The most ever said of these location-based evolutions are that certain areas give out a magnetic charge that can allow certain Pokémon to evolve, and the most said about the evolution stones themselves are that they emit a radiation that induces evolution in certain Pokémon. The in-game explanations never contradict the changed evolution methods.
Lore is a whole lot more than what's directly stated in games, especially in a series like Pokémon that focuses on its creatures. The Pokémon themselves, their gimmicks and all, are the central factor of the Pokémon World. It's a part of the world-building.
There are much, much bigger changes whenever an enhanced remake comes out--Mauville City was an ordinary-looking town in Ruby, Sapphire, and Emerald but was changed to an indoor town in OmegaRuby and AlphaSapphire, with a second-floor residential complex placed on top of it.
And not only does Omega Ruby and Alpha Sapphire take place canonically in a different timeline than the original Ruby and Sapphire, but the original games were under extreme technological and video game limitations. Limitations like those should not be taken as canon, otherwise it would be canon that bathrooms don't exist in the Pokémon world.
Similarly, the latter set of games have a Battle Maison that didn't exist in the original Hoenn games, and Kyogre and Groudon got Primal forms they didn't have before. The official explanation for the drastic differences in the regions, the characters, the mechanics, etc. is that there are multiple Pokémon universes, each of which operate under different rules. So even if it seems really weird you could use a Thunder Stone to evolve Charjabug into Vikavolt or an Ice Stone to evolve Eevee into Glaceon, there is still that explanation that Sword & Shield could be set in a different universe than the other games. After all, Zinnia does speak of universes where Mega Evolution does not exist--these games might be one of them.
So you want to make the claim that Sword and Shield takes place in its own timeline simply because some Pokémon evolve differently? Seems pretty drastic to me, especially when there are no other indications that these games take place in their own timeline.
Functionally (that is, from a Doylist perspective), this was most likely done so they wouldn't have to shoehorn a mossy rock, an icy rock, and a magnetically charged area into every region and make sure they're located at the right parts of the games that those Pokémon should be allowed to evolve. There were issues in the past in which FireRed and LeafGreen rendered Espeon and Umbreon unobtainable in-game due to a lack of day/night cycles (the Orre games addressed this by just alowing you to catch Espeon and Umbreon), and these are issues Game Freak probably didn't want to do again. The Pokémon games are already a MASSIVE victim of feature creep, and location-based evolutions are likely a lot of work from a design perspective. (Probably not so much from a programming perspective, but the people who design the regions themselves would have an increasingly tough job as more location-based evolutions get made.)
That's the thing though, they're not difficult. Every Pokémon game has an icy/snowy area. Every game has a forest, every game has some sort of power plant or electrical area. Sword and Shield included. Maybe (and that's a strong maybe), if these were games with no obvious place to put the evolution areas, I wouldn't mind as much, but the areas are clearly there, it would not be difficult to use them.
Grubbin/Charjabug/Vikavolt is Alola's regional bug and a readily accessible Pokémon in Alola's early going - I would question where the logic was of making its final evolution unobtainable until one of the final areas in the original Sun & Moon.
I question the logic too, but the solution to that would have been to either not have Vikavolt evolve like that from the beginning, or to find a different electromagnetic area, like they did in Ultra Sun and Moon.
It's not logical to keep hanging on to a needlessly inconvenient gimmick just to preserve some sort of "consistency" or "canon". Pokemon is a video game and video games get updated with new releases.
And it's also a series with massive world-building. And if you have this world-building, it should be the goal to remain consistent if possible. World-building is an important part of video games like Pokémon.
Plus...just like using the Lapras you are supposedly riding on in battle never made sense,
That's a video game mechanic, not a function of the world. Sometime things have to not make sense in order for the video game work, there's plenty of examples of such things in Pokémon. Different issue entirely.
In Gen one we could already love our Pokemon and treat them well...yet no Golbat ever evolved into Crobat... There were no Sentret on Route 1 (in GSC there suddenly were) and NPCs kept saying that there were only 150 Pokemon, when they really should have known about all the others.
Or why couldn't my Vulpix learn Will-O-Wisp in Gen 1? After all I got it to level 24, so it should have learned it, shouldn't it?
I know this isn't the best response, but I would argue that we really shouldn't be looking to Gen I for examples. Game Freak very clearly was still figuring out what they were doing in the first two generations, and it's not really until Gen III that we started getting consistent world-building.
What's so special about some crummy rock with moss on it anyway that a Leaf Stone shouldn't be able to to?
That it's a completely different object with completely different effects on Pokémon, perhaps.