Should DOS be added to platforms?

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No, for the same reason we don't have Windows as a platform. The "PC" platform is a catch-all. Having a separate DOS platform would just result in duplication because of the amount of DOS games which are available for Windows (directly, or indirectly through bundled Dosbox or game specific wrappers).

Linux is its own platform because it's a multi purpose operating system that runs on different classes of hardware many of which are not PC's.

Yeah, but a lot of games are DOS-only. This can help users searching with filters.