I’m finding it hard to understand how this new feature differs from Selective Sync.
Is there something this lets one do that couldn’t be done before? Or is it just adding convenience?
This is kind of the opposite. With Selective Sync, the file exists on Dropbox.com and 0 or more client devices, and each device can choose whether or not to sync the file. The new ignore feature allows a file to exist on one device without being uploaded or synced to other devices.
Try unselect node_modules in Selective Sync for ignoring purpose. Uh-oh, you just deleted node_modules in your machine.