I have seen an example of the brush used to draw a straight line with no size to the brush, so its shape was all feathering. I don’t see anything in the manual as to how to do that. Does it take a drawing pad to do that?
See milky way processing in darktable, looking for feedback - #7 by jdneumeyer, jdneumeyer Jordan Neumeyer’s xmp.
You can set brush smoothing to ‘low’ in the preferences (which converts the brush stroke to a line with very few nodes) and delete any superfluous nodes by right-clicking on them. Scroll to set the ‘hardness’ (which adjusts the feathering).
@Underexposed Thanks for posting this question! I have been a bit frustrated when using the brush and needing to zoom in and delete about 20 of the 25 or more nodes created.
@elstoc Thanks for pointing out the option to reduce the number of nodes in the preferences!
It would be nice to have a straight line with only 2 nodes, perhaps by pressing the shift key like Gimp does? Just a thought.
You can achieve the same thing with a path shape - which is probably the better way to achieve it. Just draw three nodes while holding Ctrl and place the first and the last node very close to each other (almost in the same place).
Thanks! I just tried that out. I hope I’ll remember that when I need it. 
Line removal (as in power lines and such) would/could be easily accomplished by simple side-to-side averaging. We used to do this when a scanline in a satellite data set was faulty.
@elstoc Thanks for your ideas. I like especially the path shape one; I will test that out.
@davidvj I don’t quite follow what you said.
When there is a thin line (power line for instance) that needs to be removed then averaging the adjacent pixels (usually top and bottom) to provide a replacement value can make sense. This was a preferred method when an image data set showed a blank scan line that needed to be fixed.
Second option in the darkroom preferences tab “smoothing of brush strokes”