From @jeffhandley delivered by @buyaa-n:

Immo we wonder why the ObsoletedInOSPlatformAttribute has In where others just SupportedOSPlatformAttribute and UnsupportedOSPlatformAttribute, our messaging structure is like

This call site is reachable on {platforms list}. 'The.API' is supported on: {platforms list}

Then the Obsoleted attribute as In but the message will have on:

This call site is reachable on {platforms list}. 'The.API' is obsoleted on: {platforms list}

We might want to change the attribute to similar ObsoletedOSPlatformAttribute or there is specific reason of having In?