This story has been labeled as a RUMOR. As a rumor, the following information should be taken with a grain of salt until we hear from the people involved.
Marvel Studios has been very tight-lipped about what movies are going to be in Phase 4 of the Marvel Universe. Kevin Feige has hinted that Doctor Strange 2 is one of the movies that we'll be seeing in the post-Avengers 4 world. The Mirror seems to have some new details, though they are by no means confirmed.
The first is that the production for sequel, which has been mentioned but by no means confirmed by anyone, is set to begin production this coming spring. Marvel would have to make some sort announcement about the movie sometime soon because it would really hard to hide a production like this. That not only means confirming the movie exists, but also confirming a script and a director. Thus far there haven't even been any rumors about Doctor Strange 2 aside from the fact that it's one of the movie that will probably exist.
The second is that star Benedict Cumberbatch is getting a raise. According to the source Cumberbatch is expected to bring in £7.5 million, or just over $9.5 million in US dollars, to appear in the rumored movie. The source notes that the filming of the sequel would also set another season of Sherlock back another year, with 2020 being the earliest.
"But the nature of the shoot for the sequel means we're looking at 2020 for the next time we see the sleuth in a full series – if it happens at all."
We're still waiting for Marvel to announce their next slate of movies after Avengers 4. We know about Spider-Man: Far From Home currently in the production, but even a confirmed movie like Guardians of the Galaxy Vol. 3 is up in the air after James Gunn was fired.