How do Legacy Skins work?

I've recently stumbled upon a skin that I absolutely adore, [Blacksmith Poppy]( Unfortunately, it's been put into the legacy vault _way_ before I even heard about League of Legends. Money is (almost) no issue, I just want that skin, but I got no idea how it's possible to get it anymore (apart from dumb luck when opening a crate). Does anyone know? Also, how frequently do legacy skins get available again (because it mentioned that they aren't gone forever, unlike Event skins) and in which way?
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