It's raining and cold this weekend and I just made some split pea with ham hocks that was fantastic with a hearty 30% whole wheat, 10% rye and 5% buckwheat sourdough boule (I don't have a name for this bread, I just made it up).
Adding other flours to sourdough adds more depth of flavor and I find plain white sourdough kinda boring.
I like a thick crusted bread with a nice chewy interior to accompany soup.
The interior serves to mop up delicious broth and a nice chewy bread holds together while others might break up on saturation with broth. The solid crust serves as a robust handle for delivering the soup soaked bread to my yap hole.
But you can pick a flavor for your bread depending on what soups you have.
That said, you can really pair flavors however you like. You might like a rye bread with something like french onion soup, but not with a creamy carrot soup. You might like sourdough with chicken noodle but not with mushroom bisque. It's really up to your flavor preferences.