The only way to get them is rare drops / rerolls from chests. They already made it pretty clear, months in advance that they won't be bringing 2018-2019 prestige back for direct purchase (and that hasn't changed)
As someone that owns all of them, I honestly don't see the demand. They aren't that special. Just a recoloring of the same skin you can buy, with some small model changes and kind of cheap splash repaint. If they truly wanted them to be special I think they should have had a 100% brand new splash for starters. Even their other variants should have done that (Sorakas legendaries feel sad having one that represents dawn and then the same one represent ing night with just a recolor in the same pose that doesn't even fit that type of character).
Then in some cases they could have really pushed the model and effects changes to really bring them up a notch vs the original.