Currently a backup from an old system, restored to a new one, will not recreate the indexes with the current index version. As older index versions are deprecated this could become more than an annoyance.

Mongorestore should strip "v" when inserting into system.indexes.

There may be some corner case when you want to keep the old version of the index, but I can't think of one. If so an option can be made to allow mongorestore to propagate the "v" field.