Robert Henry wrote:

Well, since he cancelled IFR it doesn't matter.


Well, it would be nice if it did, too. It remains my hope that a midair in
Class D airspace because the inbound aircraft was instructed (verbally or
through omission of other instructions) to continue, and the outbound
aircraft was proceeding on the inverse (ODP) heading is not going to be a
career enhancing incident for the tower. In other words, "How did you not
know that the departing aircarft was flying the ODP."

But he cancelled, so the tower controller is under no obligation to do
anything. When I worked at the VFR tower we would simply say "traffic
is one Cessna inbound from the NE, freq change approved."


Is it safe to conclude that if an aircraft departs IFR without a release
that the ODP may not be protected at that time?

Sure, it may also be littered with VFR aircraft going the same direction.


I am certain there is a
relationship, even if there are 100+ better reasons that most SOPs/LOAs
require a coordinated release before departure.

All towers have an LOA with their approach control on how they will
handle IFR traffic. Some class D's will have automatic releases on a
few headings or within a pie. The pie will be some wedge fanning out
from the departure end encompassing about 60 degrees. Most class D's
will have to call for each release. Likewise the approach control will
be required to inform the tower of each inbound and turn over comm by a
certain point. Now if all IFR aircraft will be flying a certain same
exact procedure then what you have is essentially nonradar airspace.
When that happens there will only be one aircraft in the area at a time,
whether that is a departure or an arrival.