It says "citation needed" for that part of the wiki article, so if you've got one, go ahead and add it in. I think it's said that for a while, so I am starting to suspect there is no canonical reference to that principle as regenerative shielding and that this represents mere speculation.
I don't really think of the Borg having shields that weaken and then regenerate. In their early appearance, I'm not sure they used shields per se at all on the ships (later, ST:Voyager seems to have them using shields more similar to what we're used to, though this may have been some sort of conversational shorthand). The whatever-shielding they use on drones doesn't seem to kick in until they have a read on the weapon, and afterward seems to be proof against that weapon, or so tough to batter through that Our Heroes don't typically view it as an option.