unit_root_vars should be made obsolete
The unit_root_vars command should be removed.
For a model with unit root vars, it is up to the user to provide a steadystate file, and Dynare cannot check its validity.
The "check" command should probably have a flag to tell it not to recompute the steady state (or that could be a flag on the "model" command).
Note also that there is still some code in estimation which uses options_.unit_root_vars.