The subway itself definitely belongs in OSM as railway=abandoned and ideally man_made=tunnel. The stations are iffier, but at least general-audience maps won’t mislead their users if you use abandoned:railway=station instead of railway=station.

There are actually two OpenRailwayMap sites these days. The one at https://www.openrailwaymap.org/ is no longer maintained and doesn’t support any tagging scheme for abandoned stations that I know of. The more modern one at https://openrailwaymap.app/ doesn’t show the Cincinnati Subway at all, because it gets its historic railways exclusively from OHM, which doesn’t have the subway yet but really should.

This subway is a bit of an oddball, since it was only ever under construction. In OHM, we’d probably end up mapping railway=construction from the 1900s to the 1920s, followed by railway=abandoned (plus man_made=tunnel). The station would similarly be construction:railway=station followed by abandoned:railway=station. If the newer ORM doesn’t show the stations with these tags, then we should be able to get the developers to fix that much sooner than with the older ORM.