One of the questions about the mobile first index from SMX East last night was about how the mobile first index rolls out. Gary Illyes from Google said he thinks it rolls out at the host level. So www.domain.com vs m.domain.com vs something.domain.com etc etc.
So if you have subdomains, Google might push them out individually. But it seems Google is not rolling it out on a page by page level. Instead it is done at the host domain level, which is a much higher level than the page by page level that we would get with a Penguin algorithm or HTTPS algorithm, for example.
Here is a tweet from the talk:

To my knowledge Mobile First Index rolls out at a hostname level.-@methode #SMX
— Dan Leibson (@DanLeibson) October 25, 2017
He was pretty confident about his answer but was not 100% sure.
Forum discussion at Twitter.

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