In a full manual lens, it should be easy enough to offer circular "aperture" at all apertures.
what role if any, circular apertures play *wide open* (i.e) at f1.2 in 85mm is debatable.
Circular blades or more numerous blades may have more of an impact in OOF as you stop down...specifically in rendering more pleasing spectral highlights...think of those geometric pentagons one sees with the nifty fifty...
Wide open...say with 85L at f1.2 or 135L at f2... Blade curvature shape may not be a major contributor. 135L has just regular blades. As one stops down yes it is useful...But both these lenses are used most often wide open.
On the other hand, intentional SA may have a lot to do with why 85L "holds" the bokeh longer at relatively the same level even as you stop down to some extent while the competitors start to show harsher and harsher bokeh...of course as you hit ~ f2.2 or ~f2.8 this advantage is lessened... But One doesn't buy an 85L to use it at f4.0
The trick is not to let the SA affect the sharpness, or find a balance... and this is where the canon designers did an excellent job with the 85... it is a sharp lens.