A rare LoL skin is one Riot stopped distributing years ago. There are three ways a skin becomes unobtainable: a one-time event sale that ended (Worlds 2012 Riven), a ranked reward tied to a single season (Victorious Jarvan IV from Season 1), or a promo code given out at an event (PAX Twisted Fate) or bundled with a Collector's Edition of the game (Black Alistar, Young Ryze). Riot tags these as Limited skins, sitting outside the regular tier system you'll find in our full LoL skins catalog. You can't buy them with RP, pull them from Hextech, or earn them back through any in-game grind.
The single event that locked most of these skins permanently was July 11, 2014. On that day Riot deactivated every promotional skin code ever issued, including the ones from the 2009 Collector's Edition pre-orders and every PAX convention code from 2009 onward. Anyone who hadn't redeemed by then lost the chance for good. The skins themselves stay on accounts that already redeemed them. Riot has never gone back and taken a Limited skin away from an account that owned it.