Some of the functionalities that the Apple Watch had in play have been transferred to the so-called Adventure Sync function. This consists of retrieving fitness tracking data from Google Fit and Apple Health. This counts towards earning buddy candies and hatching eggs. The more activity is logged in the apps, the more progress. The advantage of this is that the Pokémon Go app does not have to be constantly open and therefore uses the battery charge.
Pokémon Go received support for the Apple Watch in December of 2016. The functionality was even announced with some fanfare during an Apple event a few months earlier. 1 in 3 respondents to a Pokemon Go Hub poll say they will miss the added value of the Apple Watch app.