Should I use a 2D or 3D dimension for an 8-bit RPG game in Unity? I have been searching and I haven't found anything on this yet. Can anyone please tell me the better option because sometimes people recommend to do 3D just in case, but I don't want to mess anything up later on.
It doesn't really matter that much what project template you use, because it actually changes very little. Anything it changes can be changed back after you created the project.
And it does not lock you out of any features either. You will have both the 2d and the 3d features of the engine available, no matter if you choose 2d or 3d when you create your project. You can in fact mix and match sprites and meshes in the same scene without issues. The only part of the engine where 2d and 3d are separated are the physics engines. 2D colliders won't interact with regular (3D) colliders. But they can still coexist in the same scene.
For more information on this topic check out the article 2D or 3D projects from the Unity Manual.
That being said, for a game where environment and characters are represented by sprites and not by 3d models, you likely want to start with the 2d template, because then any image files you import will by default be imported as sprites and not as textures. You can change the import type of an image asset very easily, but it's still more convenient to have the right preset.