All the few gliders I flew have and additional placard listing the Vne changes at various altitudes. The ASI mark is at the Vne at sea level.
Mine goes from 275 km/h at sea level to 210 km/h at 8000 m.
The Vne calculation is quite complex and involves both IAS and TAS, thus the need for a table of Vne at various altitudes arises if you don't want to use the lowest one.