Selecting Hairs in "Particle Edit Mode"
With a point selected in the "Point Select Mode" of the "Particle Edit Mode" of the "3D Viewport", you can select the whole hair in Blender 2.90 with CTRL + L ("Select Linked"):
With a point selected in the "Point Select Mode" of the "Particle Edit Mode" of the "3D Viewport", you can select the whole hair in Blender 2.90 with CTRL + L ("Select Linked"):