Google's John Mueller said on Reddit this morning that he believes that all sites running a separate mobile URL for their mobile site solution should work on moving those implementations to a responsive site design instead.
He said that mostly because it makes it easier to maintain, not because of any SEO benefit.
John wrote "at some point all of these sites with separate mobile URLs should just move to a responsive design anyway, which makes all of this moot. (Separate mobile URLs makes everything much harder than it needs to be)."
Here is the full quote:
With mobile first indexing, I'd recommend integrating the mobile version just as well as you would the desktop version. That means adding the alternate-amphtml link to the mobile version (pointing at the amp version), and including a reference to the mobile version from the amp version. That said, at some point all of these sites with separate mobile URLs should just move to a responsive design anyway, which makes all of this moot. (Separate mobile URLs makes everything much harder than it needs to be)
Again, Google supports responsive web design, dynamic serving and separate mobile URLs. But again, they always have recommended to go with responsive because it is cleaner and easier to maintain.
Forum discussion at Reddit.
