I think there's a bit of confusion here. The viaduct is, as has been said, out of use - it has no tracks, it is part of a shared use path and it isn't considered structurally sound to ever be used for trains again. It's perhaps the biggest individual obstacle in opening the line further towards Plymouth.
However, the line does still exist until just before the viaduct, with the shared path running alongside it rather than in its place. It is very much in use for freight operations from Meldon Quarry, which has seen increased use in recent years for HS2 ballast. There are clear remnants of the heritage railway operation at the former Meldon Viaduct station, including the platform with shelter, and signs and buildings indicating that a cafe and toilets were there in the recent past. Other than the platform itself it unfortunately looks very dilapidated despite being in use until very recently. I suspect vandalism has occurred. It is a great shame as in the summer months it probably gets more than enough passers-by to justify a cafe stop, even without the heritage trains running. Although the aforementioned increase in quarrying may be an obstacle as it is on the quarry works site.
In Geoff Marshall's video on the Okehampton reopening, a Dartmoor Railway Association representative states that they hope to be able to restart heritage services within a few years (at ~ 7:30):