The "Custom Properties Slider" in Blender 2.90 can be found in the "Item Tab" of the "Properties Panel" at the right side of the "3D Viewport", if you have selected the bone (in this case "CTRL_IK_hand" which controls the "IK Chain") to which this slider had been assigned before in this bone's "Bone Properties Tab" which can be found in the "Properties Editor":
