Answer
No! Account content (Champions, Skins, RP, etc) can never be lost through inactivity. Only your Summoner Name will be affected.
How to Delete Your League of Legends Account
league of legend accounts are now riot accounts *EXPLAINED*
Does League of Legends delete accounts?
No! Account content (Champions, Skins, RP, etc) can never be lost through inactivity. Only your Summoner Name will be affected.