Michael,
I agree it no longer works the same in V12. I reported the bug to Support.
If you do not do the Descriptor system and use one of the options for StateSpaceRealization, it works:
StateSpaceModel[eqns, {i[t], Subscript[v, l][t], Subscript[v, c][t],
Subscript[v, r][t]}, {Subscript[v, s][t]}, {i[t]}, t,
StateSpaceRealization -> "ControllableCompanion"]
Also, to view all of the StateSpaceModel terms, you can click on the "S" in the upper right corner and it will expand the gray squares.
Regards,
Neil
