I think most of our historical railway mappers have joined OHM, though there’s still a lot to map and no particular deadline. The focus has been on making sure new historical railway data gets entered more rigorously into OHM in the first place.
I think OHM is a great project and well prepared to welcome historic data, also data from which no traces can be seen any more, but its existence is not related to what we map in OpenStreetMap. Everything that can still be seen can also be entered in OpenStreetMap.