Node Downstreamer is an addon that allows you to select all nodes connected to an active node downstream or upstream.

This addon gives you access to 4 actions (operators) that you activate with a shortcut. You can set a custom shortcut for each of these actions under Preferences - Add-ons - Node Downstreamer

Select Downstream allows you to select all nodes connected downstream of the active node (see picture below)

Select Downstream One Step allows you to select all nodes connected directly to the active node downstream (see picture below)

Select Upstream allows you to select all nodes connected upstream of the active node (see picture below)

Select Upstream One Step allows you to select all nodes connected directly to the active node upstream (see picture below)