At what level of resonator damage are existing links destroyed? I had heard that the portal must have more than two resonators remaining for any links to stay up, but I can't find any documentation of this.
I can bring you my experience on the field: the links on a portal are destroyed when there are two or less resonators remaining.
Sadly, the "official documentation" is more a collection of FAQs than an actual manual, so you'll have to rely on external resources or your direct experience to gather this kind of informations and become a better Agent.
How far you can link form portal depends on the total energy of the portal.
From my observations, but no calculations:
The link goes down when the energy of the portal goes below energy needed to create that link.
When attacking a portal with multiple links they get destroyed consecutively, not all at once. This supports the above theory.
The link(s) are dropped once all resonators at the portal are at critical level.
As Kappei said, usually links go down, when portal has 2 or less resonators, but, I think thats is caused by the total of energy remaining in the portal. (i'm not really sure about it).
Just remember that, you CAN'T make a link from portal A to portal B if some one has less than 8 resonators, namely, the links are made between full portals
According to Google Ingress help:
Destroy an enemy controlled Link by attacking the Resonators that are deployed on the Portals that anchor the Link. For example, you can fire an XMP to damage a Resonator and reduce its XM health. When all eight resonators have their XM health reduced to the Critical level, then the Link will fail.
Link to text: https://support.google.com/ingress/answer/2808353?hl=en
I read somewhere, that links drop out, once all the resonators are below a critical energy level. This could be why some links appear to depend on the last 2 resonators?
As in, 6 resonators have faded/destroyed, but the remaining 2 have say 20%. Critical level is 10-15%, so the link stays. The logic that ALL 8 portals are below the critical level, will fail, as the two remaining are above. I may be wrong, but thats how I read it. Keeping two charged up, should therefore hold a link. Might even work with one? But not had time or patience to test this.