Riot Games announced that they are looking to switch League of Legends' API to DirectX 11 soon and are asking players to opt into testing it on the live client.
Believe it or not, League of Legends is still running on DirectX 9, the API that went out of use many years ago. DX10 came and went but we don't speak about that one and most games are currently using DX11, which is the one Riot are looking to use for LoL.
Technically, we are all waiting to switch to DX12 soon but that one is still fairly unstable in numerous games which is probably the reason why the devs are sticking with 11 for LoL. Salty Yasuos blaming their 10 deaths on lag is bad enough, they don't need API to act as another excuse.
If you feel like testing the new API, you can go to the Configs folder which is, in turn, found in League of Legends folder. Before the next step, copy the .cfg file to another location so you have backup, just in case.
There will be a game.cfg file in there, which you can open in Notepad. Find the [General] section in the text and add "DX11BetaTest=1" without the quotation marks.
Currently, the API shouldn't make any significant changes to your League of Legends experience but it will certainly open many doors to Riot Games.
If you played League of Legends for longer than a few days, you are probably aware of at least some of the many client issues. Most of the community noticed them over the years as well and DirectX 11 should hopefully give Riot the tools they need to finally introduce the much-needed improvements.
You can check out more about the API announcement on the official page.