So I've looked into this and it seems that Instagram only notifies users when someone screenshots their story, not their posts. This is because stories are meant to be more private and ephemeral, so the app gives users more control over who's interacting with their content. When you screenshot a story, the person who posted it will see a little camera icon next to your name in their viewers list, indicating that you took a screenshot. However, this only applies to stories, not regular posts, so you don't have to worry about getting notified for screenshotting those.