Natives might have a different take, but I was recently in Berlin (which you might consider an 'established city'), and this seemed like a solved problem there. Bike lanes are at sidewalk grade, above the curb, with rumble strip or other tactile indicator for the boundary. You do NOT walk in the bike lane, it's just like walking in the car lane.

Considering the rise of additional micromobility vehicles, this is such an obvious solution when you see it in practice.