There’s obviously been a big debate going on about how Old Man Cap came about at the end of Avengers: Endgame. Did he take the slow path back to the present? Alternate timeline? Gauntlet shenanigans? Well, the Russo brothers have done an interview for Entertainment Weekly and answered that question, while still leaving a few open for discussion.
Alternate timeline. He was living in an alternate timeline and came back.
From the article...
Does Cap go back to live in the past— or does he go and live in another dimension?
The directors say it’s the latter.
The article also touches on where the shield came from, was he up to much in that alternate timeline, etc... The directors are pretty coy about a lot of it, but it’s still a good read and nice to have one niggling question answered.
Thank you to There Are Four Lights for bringing this to my attention.