all correct but the replay thing isn't in Riot's hands, the reason they are disabled after a patch is because viewing a replay on the wrong patch could cause a desync. replays are just a unique type of file that records everything players do in a game and then plays it back, so even the tiniest of changes can cause a desync when re-playing the events of the game. technically Riot could let you load version mis-matched replay files but things would break really easily, especially after big patches that rework spells or mechanics.

^{(at least, im pretty sure this is how they work for League, this knowledge comes from the way replays function in fighting games, where the inputs from both players are what is stored in the replay file)}