One of the things I read about is that moving forward, the MCU will no longer be telling stories chronologically. I think it was the Russo brothers who said it in an interview. So, there could be more stories from the past, which could explain the Black Widow movie and Thanos turning up in The Eternals. Of course, since the Russo Brothers are no longer making MCU movies, at least for now, I'm not sure how reliable they could be about Phase 4.