Even though this should be a default development technique for a mobile site setup for most developers (and SEO professionals) nowadays, there may still be some people who are unaware of this.
An SEO professional wanted to know: is there a difference between using example.com and m.example.com when it comes to Google indexing sites on the mobile version that’s on a subdomain?
John explained that there usually aren’t any issues using m-dot domains in general. This is one of the supported formats that Google can still crawl.
Even so, John doesn’t recommend using an m-dot subdomain at all, and instead recommends the more modern approach of using a responsive design.
He also mentioned that although Google is unable to see what’s on your mobile site on an m-dot, this could very well be due to issues with Google being able to access something rather than the use of the m-dot subdomain setup.
This happens at approximately the 30:07 mark in the video.