It's been explained that some retired skins have been pulled off the store due them not being up to the quality standards that Riot now has.
These skins become legacy and re-pop up here and there.
I for instance got Firefighter Tristana from a bundle they had for EU LCS at the end of Summer Split 2015, cause Rekkles got a Pentakill with that and since FNC won the split, that bundle just popped.
Riot doesn't want to clutter the store with skins that are just recolors or just not as good as latest skins.
You can always look for special sells of the Legacy skins. The limited skins however won't return, but Riot hasn't made a limited skin since Championship Riven, and that one even came back for a single person who predicted the entire 2015 World Championship and there's been talks of making it available as part of a bundle for Worlds this year.
Other than that... you could always end up being SUPER lucky on Hextech crafting
[slayer-jinx-catface]