Do revived dead batteries become less effective?
This is a major annoyance with &quot;smart&quot; chargers in that they follow certain recommendations and refuse to charge a battery that is too dead. You can probably revive the battery with a bench power supply to the point where the charger will work. Old fashioned chargers, which consist of little more than a transformer and rectifier, will also work.
According to this website, deep discharge (below 10.5V for a 12V battery) causes actual damage (sulphation of the plates) which is irreversible:

Lead sulfate (sulfation) now coats most of the battery plates. Lead sulfate is a soft material, which can is reconverted back into lead and sulfuric acid, provided the discharged battery is immediately connected to a battery charger. If a lead acid battery is not immediately recharged, the lead sulfate will begin to form hard crystals, which can not be reconverted by a standard fixed voltage (13.6 volts) battery converter/charger.

This is certainly my experience with automotive batteries in practice, especially in a cold climate.
So I think, even though you could probably salvage the battery and get a bit more life out of it, it would be better to replace it so you don't get stranded somewhere.