Fullbright is technically classified as a resource pack rather than a traditional "mod," though its installation requirements vary based on the desired effect.
If you don't want to use a texture pack, you can edit your game files to achieve the same effect: Simple Fullbright - Minecraft Resource Pack - Modrinth fullbright texture pack 1.12.2
Infrastructure on servers like 2b2t is largely built on 1.12.2. Fullbright is technically classified as a resource pack