It's believed within WWE that Brock Lesnar is focusing on his UFC return for early 2019 but that "the door is open" for a WWE return down the line, according to PWInsider. It looks like Lesnar is done with WWE for now but officials would be happy to have him back when his UFC return is over.
As noted, Lesnar had been advertised to appear at Monday's RAW in Brooklyn but he flew home after SummerSlam on Sunday night via private plane. There is no truth to the rumors of the RAW script including a promo by Lesnar and Paul Heyman at any point in the day. The backstage segment with Heyman and Kurt Angle was taped.
There is also no truth to the rumors of Vince McMahon calling the SummerSlam main event early. The Roman Reigns vs. Lesnar match went down exactly as planned. Braun Strowman was booked to be at ringside during the match to remove any potential hostility from the crowd towards Reigns and Lesnar.