SERVER-58246 outlines a race condition where a command that was marked as never allowed on secondaries and later set it's opCtx as interruptible on stepdown, may actually run on a now secondary uninterrupted. In SERVER-58246 it was decided that it was not feasible to prevent this at the command infrastructure layer.
This ticket is to prevent this race from happening on legacy (pre-5.0) DDL operations. Since the legacy DDL is not network-partition tolerant, a stepped-down former primary running DDL concurrently with a new primary may cause harm. Interrupting the DDL as soon as a node realizes is no longer primary mitigates this situation, although it doesn't prevent from happening it in the actual network-partition scenario.
On FCV 5.0, since the new DDL coordinators are tolerant to split brain scenarios, this is not required for correctness.