The relevant wiki documentation makes it sound like most areas stick to one mapping scheme over the other, and indeed, another town i've mapped in sticks to just separately marked sidewalks. https://wiki.openstreetmap.org/wiki/Sidewalks

The problem is that using both would be inconsistent from a mapping standpoint but I imagine also a routing standpoint.

I agree that separate sidewalks is more detail but it feels wrong to keep it just in the new regions of town without going back and changing the scheme for the rest of town, which would be a lot of work.