Mobile-first indexing rollout begins
Google began migrating sites to mobile-first indexing, notifying each site through Search Console as it switched.
What changed
After eighteen months of experimentation, Google started moving sites to mobile-first indexing, using the mobile rendering of a page as the primary version for indexing and ranking. Migration was gradual and site by site, with notifications sent in Search Console, and Google stressed that mobile-first indexing was not itself a ranking boost. Sites with mobile content that omitted text, images or structured data present on desktop were most exposed.
Timing
Rollout began 26 March 2018. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.