i was thinking on using it just because i find an addon that lets me render livesplit inside the game in fullscreen... If it is banned, is there other solution to that problem? (no window mode pls, it has so much input lag)

Not allowed, no. You have 3 options that I know of:

1. Get a second monitor.

2. Stream your runs with OBS (can be a private youtube stream if you want) and watch your own stream from another PC or a laptop or your phone, where you'll be able to see your LiveSplit with just a few seconds of delay if you ticked the "low latency" setting.

3. Play windowed.

You can windowed the game by pressing "Alt+Enter" mid-game