The point of a headcanon is that it's just fans thinking about parts of a story or a universe in a way they like. Here, it's shipping Bucky and Steve together and headcanoning them as bi because they already have a strong friendship in canon and adding in some romance for the only reason that well, we just think they're cute together

It's harmless to canon, and it makes for nice art. Overall, it's really just a harmless ship between two characters that already have a strong connection in canon.