This has been one material that has taught me a lot about SN1987A (a recreation of live events of when it happened, it used to be a twitter acct but now it is archived https://bmonreal.github.io/RealTime1987A/ )
The column mass (ie, the spatial density multiplied by the length of the observer’s line of sight through the material) is highest at the limbs when looking at a gaseous shell. So shells like this look like rings from all angles. A related effect called limb darkening is the edges of the sun always look less bright, independent of viewing angle
The ring is not a product of the supernova. It's pre-existing material, that is being energized by impact of the supernova ejecta (it took a few months for it to start brightening).
There are actually three rings, a smaller one around the previous star's equator, and two larger ones above and below. The hourglass-like figure is actually quite common, produced by the star rotaton.
In Alastair Reynolds' "House of Suns", the Gentian Line of spacefarers construct "stardams" around stars to prevent their supernova outbursts from harming nearby civilizations.
Given the amount of empty space out there, the amount of black between stars, it would seem more efficient to create shields around the civilizations than around the stars themselves.
"Efficient" in the economical sense, that is. Unless, the civilization covers a very wide area (basically everywhere in there, for light years around the impending supernova) and the protection amounted for each and every civilization zone would surpass the containment circle/cylinder/sphere around the star. In such case the sensible solution becomes to just recognize the area that can not be saved (no mater what) due to its proximity to the star and the limits of walling withstanding capacity, evacuate that, and set the containment all around the future explosion as close as possible instead.
This kind of "things we think are natural are actually the product of intelligences much larger than we currently can imagine" is all throughout John C. Wright's "Count to the Eschaton" sequence as well.
I probably read it three or four decades ago, along with others by those two authors and Heinlein as well. Easily confused, as I now live with a dog named Asimov!