Does Linea require an Apple Pencil?
The short answer: No, not at all!
The long answer: Linea Sketch for the iPad was designed from the ground up to harness the power of the Apple Pencil and the iPad Pro. However, not everyone owns this amazing combo and we recognize that. The good news is that you can use your finger or a third party stylus to draw on supported devices, you just won't be able to use additional features such as pressure sensitivity and tilt control.
Since the Apple Pencil isn't supported at all on the iPhone, Linea Sketch can be used with 3rd party stylus' or just your finger to sketch on these mobile devices. iPhones that support 3D Touch will also let you use pressure to vary stroke width and opacity with tools like the pencils and marker.
While there is no formal list of supported stylus, you should have no trouble using one with Linea.