Season 7 of Agents of SHIELD has made some significant revelations in regards to Daniel Sousa's life, and they have links to Peggy Carter's role in Avengers: Endgame too. The final season of Agents of SHIELD is taking a trip throughout the Marvel Cinematic Universe timeline and is using this to their advantage. Instead of sidestepping the MCU continuity like recent seasons, Agents of SHIELD is addressing the larger canon and adding new context to a variety of events and characters.
This practice is continuing with the involvement of Daniel Sousa (Enver Gjokaj), who now returns to the MCU several years after Agent Carter was cancelled. Once Sousa's return was confirmed, fans began hoping that Agents of SHIELD could tie-up some of Agent Carter's loose ends. The third episode of Agents of SHIELD season 7 continued Sousa's story eight years after viewers last saw him in the MCU timeline. Sousa has continued to climb the ranks at SHIELD, but his fleshed-out backstory also helped clear up portions of Peggy's life too.
After Agent Carter ended with Sousa and Carter sparking a romance, Agents of SHIELD reveals that their relationship did not last. There's no specific timetable given to when things between them ended, but it seems like several years have passed since Sousa last saw her. Thanks to this information, it is confirmed that Peggy and Sousa did not get married. Since Captain America: The Winter Soldier established that Peggy did marry someone after the events of the show, some fans hoped Sousa was her husband. The movies never shared who Peggy does marry, but Avengers: Endgame shows Peggy was still single in 1949 when Steve Rogers traveled back in time to live his life with her.
With the added information provided by Agents of SHIELD, more light has been shined on Peggy's life after Agent Carter. The second season of the show took place in 1947, so it was only two years later when Steve would visit her in an alternate timeline. This only gives Sousa a year or so to potentially be with Peggy, but the relationship could've lasted even less time. We also know thanks to a video recording in Captain America: The Winter Soldier that Peggy was married to someone by 1953 though. Clearly, this man isn't Sousa and the 1955 current setting for Agents of SHIELD means additional stories with Sousa (and maybe even Peggy) would come while she's married to someone else.
The question that does loom even larger now is the identity of Peggy's husband. The time travel rules of Avengers: Endgame would negate the theory that a second Captain America always existed in the MCU timeline, where Steve was married to Peggy the whole time, even though that's what the writers believe happened. Fans now know that Sousa isn't the mystery man that caught Peggy's eye, and the promo for the next episode of Agents of SHIELD promises to flesh out Sousa's story even more. With a story revolving around Sousa's death, perhaps more details about his life, as well as Peggy's, will be revealed soon. In any case, at least it's confirmed that Steve wasn't breaking up an existing relationship between Peggy and someone else when he surprised her at the end of Avengers: Endgame and lived a long life together after.