Google's John Mueller said on Twitter that he strongly recommends sites, espesially new sites, not use m-dot URLs. Google has always strongly recommended sites use responsive designs but here John is saying stay away from m-dots as a strong recommendation.

John said "Re:m-dot vs responsive/dynamic serving: I strongly recommend not using m-dot URLs if you can, especially with new sites. It makes everything so much easier."

Here are the tweets in context:

Re:m-dot vs responsive/dynamic serving: I strongly recommend not using m-dot URLs if you can, especially with new sites. It makes everything so much easier. Re:PWA -- they can do cool stuff quickly, but they're also hard to get right, especially for SEO (not impossible though). — 🍌 John 🍌 (@JohnMu) September 27, 2019

Forum discussion at Twitter.