If it's lore, invent a shield inverter that catastrophically-collapses a target shield bubble. If it's gameplay, health-pools can be rebalanced and other means of sustain can be devised. If it's graphics, change the wording to "damage-redirect" ala laser drones and mirrors. If it's CPU-based, use personal FFs, as a third health-bar.
It's gameplay, graphics, and CPU problem. Mostly CPU and graphics. Lore is the easy solution to why the tech doesn't "exist" any more, as you've noted.
(I've been reading the Uplift trilogy and there's several instances of human creativity coming up with an idea that the galactics didn't expect because it wasn't in the Great Library, thus it takes the more technologically advanced aliens by surprise, but the reason it wasn't in the Library was rather that it
was but wasn't
used any more due to a glaring defect that had been discovered a million years ago. For example, a space ship ends up with a refined coating of carbon soot from a red giant--some machine race converted it--that made the ship impervious to all forms of lasers and other heat-based weapony, by shunting the energy into hyperspace. The flaw in the coating was that with relative ease an attacker could broadcast a signal that would turn the coating into an "antenna," drawing energy
out of hyperspace and broiling the crew. Of course, the plot required them to survive, so they had another human trick up their sleeve and reconfigured their communications laser into a refrigeration laser* and dumped the heat back into space).
*Not actually a real thing, but it was another human invention used in the first book to allow humans to visit the sun. As in, actually dive down into it. Hence the title, Sundiver.