But what if boss can not enter stage if there were additional walls? Did bosses can pass through solid walls like world 4 boss or they need extra free space beyond what You see on screen like boss 2 and 3?
No, the boss won't leave its 4:3 aspect ratio, nor will your character. The limiting rays and extra graphics will only be visual, according to our vision. Even in World 2, you won't need rays; simply using stones to fill the black areas of the 16:9 aspect ratio already looks excellent. The combat arena is the same in the game engine; it's filled in for your eyes.