John Mueller and Gary Illyes, Google’s developers, have revealed several aspects for webmasters to know prior to launching mobile-first index.
They say that it is better to shift the ‘m.’ website format to the adaptive one before index launch. Thus, all content will be indexed. The fact is that currently Google does not index but just annotates ‘m.’ mobile website URLs.
They also stress that a mobile version should contain less amount of content than a desktop one. Otherwise, the website can be lowered in the search results when entering mobile-first index.
According to developers, websites will be indexed according to groups rather than all at once.