Trying to find essential data in OSM I encounter a paradoxical situation: even large cities are mapped as nodes without boundaries. I know what is written on Key:place#Mapping populated places as areas page, but still it's difficult to accept that a popular and geographically quite extensive feature like "city Istanbul" exists only as a node and OSM doesn't give you an idea what are "city Istanbul" limits. There exists a vast "Istanbul province" administrative unit which has a mapped boundary, but it is not what tourists and likely inhabitants mean when they say "I go to Istanbul [city]".
I think there is a way to give a visible and science-based shape to megapolises which boundaries don't correspond administrative division. An urban area is a scientific term for highly urbanized place as geo-socio-anthropological phenomenon. Unfortunately scientists have not reached an agreement about its boundary definition. Nevertheless this mustn't stop us from mapping urban areas, after all not all geographical features have sharp boundaries (seas, mountain_ranges, etc.). Some institutions provide economic and demographic stats for urban areas, with one or other way of urban area limits definition represented in the form of maps (heatmaps or linear boundaries) as at http://www.atlasofurbanexpansion.org/cities/view/Istanbul or even in geo-format (shape-files) as at https://www.arcgis.com/home/item.html?id=2853306e11b2467ba0458bf667e1c584. Of course one shouldn't copy these boundaries without legal permission. In the absence of such data an approximate outlining of dense build-up territory would be suitable (better than nothing).
Note that some not-osm-based map services to the search query "Istanbul" as the top result return a highlighted area outlining not the Istanbul province but city with boundaries according to "urban area" definition, what usual user most probably expects.
As an experiment, I've created a relation r10069486 "type=boundary+boundary=urban_area" with encompassing ways in 'outer' role and Istanbul city node as 'label'.