To be honest, the ending is pretty ambiguous. I found a interpretation of the ending on Code Geass' Subreddit that basically allows Lelouch to be Immortal, yet it does not contradict the word of the creators (Ichiro Okouchi). Source: link "Lelouch is dead either way. Either he is perma-dead, au reborn as R.R."
i strongly think Lelouch is still alive although i'm quite sad why the season had to end like this for some reasons when CC is talking to lelouch in the final scene it's kinda obvious she's talking to a real person and not an imaginary one plus aside from the final scene of the mysterious rider plus i think C.C will leave until the contract is complete but her and lelouch's contract was to take britania down but britania stayed mighty and she still lives so yeah i think lelouch is still alive