At this point, it does not look like there are any plans for Brock Lesnar in 2021.
As first reported by Dave Meltzer of the Wrestling Observer Newsletter, the rumored Bobby Lashley vs. Brock Lesnar match is not happening at SummerSlam. Lashley has been lobbying for that match for years but it does not look to be in the cards.
At this point, there is no match planned for Lesnar at SummerSlam or any pay-per-view in the near future. Lesnar has not been seen on WWE TV since he dropped the WWE Championship to Drew McIntyre at WrestleMania 36.
Another sign that a deal has not been worked out for Lesnar's return is the fact that there is no merchandise being worked on for him. WrestlingNews.co was told that the merchandise people have not been told to work on designs for Lesnar. If Lesnar was going to return at SummerSlam, new merchandise would be in the works by now.
SummerSlam is close to selling out so he's not needed to sell extra tickets. That plan for that show is for John Cena vs. Roman Reigns to headline and, as first reported by the Mat Men Pro Wrestling podcast, Cardi B is set to host the show.