There's a debate about that, even within the modern quilt guild (MQG).
Often modern quilts have lots of negative space. Often they use bold colors. They may use traditional blocks, but in different ways, such as making them much larger. They tend to use a lot of solids and no borders. So far this sounds much like a traditional Amish quilt, right?
This slide presentation tries to describe what makes a quilt modern. The most interesting line - "Modern quilting is like porn, hard to define, but you know it when you see it."
https://community.themodernquiltguil...Quilting_0.pdf
Some fabric designers have latched onto the modern quilt movement, and so quilts using those fabrics are often considered modern just because of fabric recognition, even though they do not fit any other definition of modern.