"We are working on getting certain event emotes into the new system and if you earned them during those events they will automatically be put in your inventory.
This isn't as straightforward as we expected but it is currently in the works"
-Thetimteemo (rioter)
source: https://www.reddit.com/r/leagueoflegends/comments/75r0z4/im_a_little_annoyed_that_old_emotes_that_i/do8av0p/
I generally feel that this is a good thing (I also like the sad amumu) but feel a bit cheated that they can't be more upfront about this. I deliberatly didn't get the arcade icon because it was a remake/copy of an icon I've owned since release and the emote explicitly said it wasn't permanent. Now I'd understand if this was an event from years ago, but this was an event about a month or 2 before the emote release,
If this would have been presented correcly it would deffinetly have changed my purchase priority, since arcade is my favorite skin line.
(I also kind of feel that the owners of the original icon could have gotten the emote aswell, we did pay rp for the original icon)