Response to @therickster2: "but can we talk about how they stressed mixing the time streams wasn't possible but then he did it anyways? like either that wasn't the same Steve or.?"
"Oh, we can certainly talk about that. I adhere to the Russo Brothers' explanation that Steve created what I refer to as the Peggy Rogers Timeline when he reunited with Agent Carter in 1949, because that's the explanation that follows the rules established by the film, and that he eventually returned to his original timeline as an old man using the multidimensional coordinates saved in his "timespace GPS." I also personally believe that Steve used his knowledge of the future to alter history in this new timeline, and that The Ancient One used her powers to keep the timeline hidden from the Timekeepers by transferring all of the variance energy it created to the Dark Dimension, where it was subsequently consumed by Dormammu."