Yeah, there's no bigger example of that than the anime. The anime, by nature, HAS to be up to date with new Pokémon, new mechanics, new characters, and other assorted details because Ash is going to have to encounter them, and often towards the beginning of the generation to establish them so they can be built up later on. The anime must put Ash in the new region or prepare him to go to the new region either once the first games of that generation come out or not long afterward. They must strike while the iron's still hot.
The thing is, it takes a VERY long time for an episode of an animated TV show to go from concept to finished product, more so for an entire season's worth, and even more so if it's a serial because the creative staff must get together and decide on what will happen and who gets what before they even start. Depending on the series, the timescale can span from months (Rick and Morty, My Hero Academia) to years (My Little Pony: Friendship Is Magic, JoJo's Bizarre Adventure). I don't know how far in advance the Pokémon anime needs, but going by other animated TV shows, the anime has to be planned out when the games that debut a generation is in a pre-beta state, perhaps even pre-alpha.
In other words, the creative staff working on the Pokémon anime absolutely MUST be privy to whatever they'll introduce in those games, long before they're available to the public. Since they have a long production process, they're probably the first people to learn about these details outside of the video game business. It's the only way to get it out on time.
The one thing that might have been affected by generations is the one medium that moves at an even slower timescale: live-action film. (Even then, it varies depending on whom you ask. Usually, a film spends many years on pre-production, such as concepts and screenwriting, but a relatively short time in production and post-production.) Though we won't know until official sources say so, it is very possible that Pokémon Detective Pikachu may have actually begun in Generation VI and needed almost the entirety of Generation VII before it reached a state the studios and distributors felt good enough to release to the public. This is the one case where a lack of then-current-generation Pokémon might be explained by this. (Morelull and Comfey I think were the only Generation VII Pokémon. Comfey was solely in the background, while Morelull's role could've been replaced by any other biome-appropriate Pokémon.)
