https://www.polygon.com/2018/10/25/18022886/pokemon-go-adventure-sync-walking-feature

A major new feature is on its way to Pokémon Go: Adventure Sync will at long last allow players to continue tracking their walking distance without having the game active or a Pokémon Go Plus tracker.

This is a huge relief for anyone invested in hatching Eggs and collecting Pokémon buddy candy, both of which require logging a lot of steps. If you have a huge array of 10-kilometer Eggs you’re trying to hatch, for example, now you can do it much more easily — the game can be idle or outright closed to do so. The update should make a helpful impact on our phone batteries and data plans.