The lack of a full Pokédex in Pokémon Sword and Shield is a conversation well past worn out. Despite many fans' passionate pleas, the games have already released with less than half the known Pokémon. The best hope for pro-Pokédex fans now is that Game Freak could eventually decide to add more Pokémon in with patches or special events. That hope is not entirely unfounded, as dataminers have found models for Pokémon that aren't catchable in Sword and Shield's current build. Now hopeful trainers can also add the discovery of a certain item to the evidence pile.
The Pixie Plate can be found on Route 8 in both Sword and Shield. By itself, the item doesn't mean much. It just powers up Fairy-type moves. Things get interesting when you consider the origin of the item. The Plate items were introduced in generation four to go with a Pokémon called Arceus. There's one for every type. Any Pokémon could use them to power up moves, but Arceus had a special use for them. It could use the plates to change its type. This unique function gives the Plates an unmistakable connection to Arceus.
This held true all the way through generation seven. As long as a player could get an Arceus in their game, they could use the plates to change its type. Generation eight (a.k.a. Sword and Shield) bucks this trend by not allowing Arceus. It can't be caught in either game, nor can players transfer one from an older generation game. This leads to the question of why the Pixie Plate exists in Sword and Shield when Arceus isn't there to make use of it.
The most hopeful of players can take it as an indication that Arceus will eventually come to Sword and Shield. Perhaps Game Freak added the item in preparation of Arceus' arrival. It could have even been a later inclusion made after the backlash to the national dex removal. This could be a sign that the developer is listening and plans to add more Pokémon.
Sadly, a deeper look into the Pixie Plate reveals that this probably isn't the case. Bulbapedia notes that, as of generation seven, the Pixie Plate is the only item that boosts the attack power of Fairy-type moves. All the other types have several items that do this. It could very well be that Game Freak put the Pixie Plate in Sword and Shield for the sake of balance. This arguments holds up when you consider that none of the other type Plates made it into the games. It's very likely that Game Freak put the Pixie Plate in Sword and Shield that they didn't have to make a new Fairy-type item.
Still, there's always hope that our Poké-god will swoop in and save us from Dexit. It's a thin hope, but it's there. All we have to do is pray to the giant ring horse thing.