Moving mobile websites from m-dot domains to responsive web design is something that Google’s John Mueller says very often in his Tweets. After removing your m-dot domain, you should redicte the whole subdomain to the new responsive website:
It doesn't matter (404/410 for robots.txt=crawl everything, which is fine too), but I'd keep it simple by redirecting the whole subdomain.
— ? John ? (@JohnMu) January 16, 2020
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