Unfortunately, it is not allowed to change this logo without optifine. And nowadays no one use optifine on newest versions and sodium also didn't give the ability to change that logo neither. But i think it is a good idea for this mod to adding this feature, so, please, think about it.

Open the config folder and change the square_logo.png to whatever you want. Probably have to rename it to that though. Then go into mod menu if you have it and there should be an option to use the "square logo", which will then use the custom logo.