Is that like "guides"? So its called in illustration applications, eg. inkscape. QCAD offers "restrictions" (EH, EV), but such guides will make learned illustrators happy, even if they are part of the drawing and have to be deleted after use.

yes like guides -auxiliary "construction" lines, very useful for prep work etc. you can assign them to a layer and then turn off the layer when you need to!

Actually, for me what would be good is the option to set the length of construction lines.
I really hope that this additional line tool will be implemented as I personally use construction lines constantly when drafting!

For me an indispensable would be to quickly draw a construction line on top of another line (ie. copy source line + paste in place + change pasted line to be a construction line in one step).