From Portlandmaps. blue outline is private property. The area of vehicular camping interest is the rather large space between the private property and the road, which is a considerable width, as can be seen.

City of Portland PBOT indicates this area is owned by the county but maintained by PBOT. Please advise if vehicle camping and abandoning autos (which PBOT says would not be enforced) in the space outlined in red box in the first picture is enforceably illegal.

This is not an issue of boundary lines.
Portland City Code (§16.20.130.E) requires that vehicles not park in front and 10 feet on either side of a mailbox.

MAILBOX CLEARANCE SIGNAGE APPLICATION
https://www.portland.gov/sites/default/files/2019-02/mailbox_clearance_feb2018.pdf

PBOT needs to enforce their city code.

The mailbox in that image is what matters… Not boundry lines.

They said; “they're not enforcing abandoned autos in places that do not have restrictions”.
Portland City Code (§16.20.130.E) is a restriction.

Mr. Gregory Epkes, in the context of "do not have restrictions", they meant they're not enforcing areas that do not have posted hours or restrictions, such as a meter, hour duration, Area Parking Permit, truck loading and such specific restrictions posted.

Technically the city code says every vehicle parked on any street parking shall move every 24 hours. That is something that is usually not enforced. So, the spot shown here would be a reasonable alternative to vagrancy related parking violations occurring in downtown or close in neighborhoods where posted signs are present. Perhaps with the specific concern you raised, a reasonable compromise is to allow vagrancy activity parking in this affluent smug neighborhood with a 10ft buffer around the mailbox.