On an iPhone 7 Plus, the maps on citybik.es looks terribly pixilated. I am getting a feel that either openstreetmaps does not properly support retina tiles, or no one actually bothers to use them. Not sure which is worse.
OpenStreetMap.org's tileservers don't supply retina tiles. But OSM's tileservers are effectively a tech demo and a resource for mappers, not a user-facing service. If you're building a consumer-grade product, you're expected either to use OSM data to render your own tiles, or use a third-party provider which provides tiles based on OSM data (such as Thunderforest, Geofabrik or Mapbox). Leaflet, as the client-side JS part of your mapping solution, can happily show retina tiles from such a provider or your own tileserver.
As a note, these are not OSM tiles but mapbox. Last I checked I couldn't use vector tiles from mapbox using upstream leaflet, so decided against it for now until I find a better solution.