I can only find one study that looks at this and the results were:
Effects of heel flare
Only one included article (Table 3, Figure 2) investigated the effects of heel flare construction (lateral heel flare of 25°, no lateral heel flare 0°, rounded heel) on running biomechanics. However, there were no significant differences in tibiocalcaneal and ankle kinematics (initial inversion, maximal eversion velocity) among heel flare conditions (Stacoff et al., 2001).
From Stacoff et al (2001):
It was expected that the large lever of the flared shoe sole would produce the largest eversion velocity and that the decreased lever of the round sole the smallest. This was not the case and is in contrast to previous investigations using shoe markers, where it was concluded that prominent lateral heel flares cause an increased initial eversion or eversion velocity . It was further expected that the flare of the shoe sole would increase maximum shoe eversion and the round shoe sole would decrease maximum shoe eversion. However, all subjects had the largest maximum eversion with the round shoe and the smallest maximum eversion with either the flared sole or the straight sole. It is speculated that i) the prominent lateral flare compressed considerably during touchdown hereby reducing the acting lever and that ii) the hard outer sole of the round shoe deformed very little, which may have favored a rolling action of the foot, resulting in a large maximum eversion. Future investigations may want to establish the change of lever length over time of various shoe sole modifications to clarify this issue.
I am not stating that you are incorrect – there are likely to be further additional studies, this one I have cited was a small study and looked at running, not walking, biomechanics but I am always wary when someone makes definitive statements (especially if a shoe company is involved).
I am not not looking for an argument, I am genuinely curious about this, having a couple of pairs of shoes with undercut soles.
If you have any studies I would welcome the links.