The continuity this series has literally been the best it ever has. Not even AG was this good with it. You still get those small mentions of past events like Ash and Pikachu reminiscing about their battle with Tobias, or Lance being a member of the G-Men, but now we get characters actually appearing and adding to the development of the current cast, including Ash.
With the Alola gang we learned Lillie is still traveling, Burnett had the baby, and Kiawe helping with Goh’s development. I really disliked the Alola crew, but I really liked Kiawe’s portrayal in that episode because you could really see the results of his Kahuna training with Olivia. The guy was confident, powerful, and did well testing Goh’s resolves as a trainer. Exactly how a Kahuna/Gym Leader should be portrayed.
Iris not only showed the fruits of her labor, but inadvertently helped Ash and his bond with Dragonite. Gary continued to show his constantly improving work as a researcher, his amazing skills as a trainer, and pushed Goh to further himself with his own goal. And Dawn is literally the reason why Koharu is becoming one of my favorite characters this series, showing her the joys of adventuring and bonding with Pokémon.
I could go on and on but you get the drift. The continuity isn’t used for just shameless marketing like some users would make you believe, they are used to add on to the development of the existing cast, which is what character returns should be used for.