Actually, they used to have gender restrictions on a few pieces. And given how min/maxy their builds are, it actually made a minor difference in some cases.
But I think that might have been Demons Souls, which isn't technically Darksouls.
By the later set of ogres in the Things Betwixt, somewhere behind the stoned petrified mob, there is a sarcophagus on the shore. Climb in and your character will climb back out gender swapped. I know there have been transgender players intentionally creating a character with their assingned-at-birth gender just for the catharsis of getting to transition so easily in a world that doesn't give a shit either way.
A random, obscure feature, but somehow they opted to create it and I love the implication that someone cared enough to write the logic.
I recall that the first DS has slightly different running animations with different lengths of startup / stop animations, which matters in a speedrun context. I don't know whether that has transferred into the sequels. It certainly doesn't seem like an intentional advantage, nor does it matter much unless you're really concerned about saving a few frames.