What are "downstream models"?
Downstream models are simply models that come after the model in question, in this case ResNet variants. Models for various topics within the computer vision domain often use a backbone to extract features from images, after which a downstream model is used to help to fit the model better to the task at hand. Tables 5, 6, and 7 in the linked paper give a good overview of the different ways backbones are often combined for topics such as object detection and segmentation.