Would a substitution with other milk-based ingredients be okay? Or do you want to avoid using milk-based ingredients altogether?

Would sort of bread is this for? Some types of bread don't need any dairy products whatsoever (i.e. 'use water'), while others don't need milk but would need e.g. buttermilk instead, which I assume would need to be replaced here

@Zerotime I've made some edits. Milk and milk derivatives are problematic. This is a sourdough (edited that in) and the first sourdough recipe I've used that called for milk. This produced a richer bread (especially with two eggs in the dough plus an egg wash), but still has a sourdough starter, not dry yeast, as its leaven. I don't know how much half a cup of milk contributes to that richness.

@Mithrandir24601 ^^^. (Sorry, didn't notice at first that the two comments were from different people, so wrote an aggregate reply.)