As Pokemon moves onto a new console and generation, some of its older systems are being shut down. The Pokemon Global Link service will be shut down on February 24, 2020.
For those still working their way through Pokemon Sun and Moon, or Ultra Sun and Ultra Moon, this means any gameplay feature directly tied to the Pokemon Global Link service will no longer be accessible. This doesn't affect general online functionality though.
Users on various forums have gone into detail about what this means for 3DS players. Basically, the Global Trade System, or GTS, should still be functional.
The main change comes in competitive battles. Ratings Battles, as well as your past battle history and rental teams, won't work anymore. Global Missions will also go offline, though they're already finished. Basically, this only affects you if you were looking to keep playing online ranked battles in Pokemon Ultra Sun and Ultra Moon and get certain rewards out of it. The main functionalities will continue to exist, as far as we can tell.
This won't affect Pokemon Sword and Pokemon Shield either, which will likely start to establish its own online battling presence, seeing as it's the modern Pokemon entry. But that's pretty typical for games, especially in the era of "forever games," to entice you to pick up the new entry with more online rewards. This is also likely indicative of Pokemon Home's presence in the Poke-ecosystem.
If you're currently playing Pokemon Sword or Pokemon Shield and looking for some of the tricky ways to evolve Pokemon like Alcremie, be sure to check out our guides section.