When to add salt in sourdough bread mix?
The salt doesn't kill the yeast but slows it down. As well as lacto bacteria producing the acidity.

There's even a name for the conscious adding of salt to the sourdough in German: Monheimer Salzsauer-Verfahren (in English):


  The salt inhibits yeasts and acid formation. The maturation is delayed
  with intensification of the aromas. At the same time, the processing
  properties of the dough (influencing the proteins) improve. The bread
  gets a balanced taste with fine pores of the crumb .
  
  Especially for the hobby bakery, this type of guide is suitable to
  produce breads with a balanced, mild flavor and flavor, without
  complicated and time-consuming work processes (as in the classic
  three-step guidance ).