Any plans you've seen are aspirations from private individuals.
There's no government/TfW plan to reopen that line.
That's not quite true. The corridor through the new Plasdwr development "has been designed to safeguard a route for a South Wales Metro service" according to the developers own design and access statement and it also features in the council's LDP - a plan that was approved by the Welsh Government.
I suppose you could regard Cardiff Council, Redrow and a Government approved LDP as 'private individuals' if you really wanted but the plan has featured in the Metro propsals from the beginning.
It's true the line doesn't feature in the current TfW franchise deal but then nor do the Hirwaun extention or passenger services on the Swansea District Line, both of which might well happen within the current franchise period. The Welsh Governmend deliberately designed to the TfW franchise to be flexible so that additional lines and servces could be added to it during the course of the franchise. It works very differently from the very rigid franchises awarded by DfT.