In most cases, no. American fiction is focused on the main narrative and doesn't commonly incorporate post-credit scenes. However, there might be some rare exceptions in experimental or unconventional works.
It's hard to say for sure. Different scary stories might have different approaches. Some might include an end credit scene for added effect or to provide some additional context, while others might end abruptly without one.
Sure, Toy Story 4 has an end credit scene. It could be a funny moment, a hint at what might come next, or just a cute bonus for the viewers who stick around.
A post - credit scene scary story might involve a cursed object. In the main movie, the object seems harmless, but in the post - credit, it starts to glow and strange noises come from it. It then causes chaos in the empty theater where it was left. Also, there could be a post - credit scene where a character finds a diary that tells of a horrible past. As they read it, shadowy figures start to appear around them, and the words in the diary start to bleed, creating a truly terrifying moment.
It can be quite important. It might set up future plotlines or introduce new characters. For instance, it could be the start of a new arc for a character like Qi'ra.