In most cases, no. Usually, there's no built-in notification feature to let others know if you screenshot their story in 2020.
Generally, no one can find out if you screenshot their story in 2020. The platforms themselves don't provide this information. But always be mindful of privacy and ethical use when dealing with others' content.
Typically, people can't tell when you screenshot their story in 2020. Social media platforms don't have a default setting for this kind of notification. However, there might be some third-party apps that claim to offer this functionality, but their reliability and legality could be questionable.