In QGIS 3 it is possible to extend an existing linestring by clicking on the 'virtual endpoint' cross marker as described in https://docs.qgis.org/3.10/de/docs/user_manual/working_with_vector/editing_geometry_attributes.html#digitizing-an-existing-layer
When extending a linestring along an existing feature, tracing seems not to work. Seems that I have to digitize along the feature by snapping to each of its vertices as shown here (the dotted line should trace along the thin black one, but doesn't):
This is a) time consuming and b) a source for errors.
I wonder if tracing does not work when extending a line string, is this not possible or am I missing something?