Yes, it is free to use. Check out the tutorial. Although it is free, I appreciate any small donation:
You will need a key to use the overlay. You can get the key by logging in with Twitch in the Tutorial. If you have any problems, you can contact me on Twitter @deadofarceus or via Twitch whisper also @deadofarceus. Gerne auch auf Deutsch :)
Legacy Mode is a feature that allows you to use the very old league Tier symbols (Diamond, Master, Challenger etc.).
Just message me over Twitter @deadofarceus so we can talk about your ideas.