Google's John Mueller said on Twitter that there is a delay currently with the notices that go out after a site has switched over the Google Mobile First Indexing.
John said "We're a bit behind with messaging of the mobile-first indexing because we're switching to a model that reduces the number of messages owners of many sites get (currently in test, should be catching up in the next weeks, assuming all goes well :))."
We're a bit behind with messaging of the mobile-first indexing because we're switching to a model that reduces the number of messages owners of many sites get (currently in test, should be catching up in the next weeks, assuming all goes well :)).
— 🍌 John 🍌 (@JohnMu) December 19, 2018
As you know, over 50% of the search results are indexed through mobile-first indexing now.
Like i said then, there are other ways to check if you are in mobile-first indexing. A fast way is to use the URL inspection tool.
Forum discussion at Twitter.