It depends. Some years ago I salvaged a 12V Sonnenschein SLA from some old equipment.
It read 0V on my meter, and connecting it to my ancient (1960's) non-smart trickle charger, resulted in no detectable current flowing. A colleague who had formerly been a chemist involved in battery research at GEC, said that, if it had become flat due to non-use rather than from discharge into a load, connecting it to a charger via a high resistance might revive it, although it might take weeks. It only had a nominal capacity of 7.5Ah, so I tried connecting a 12V 2.2W bulb in series to limit current. It did work, the bulb starting to glow dimly after a week, and it eventually restored about 5Ah, but it did indeed take many weeks. Not really an economic proposition unless money is tight (as it was for me at the time!). Reviving a completely flat battery, probably wouldn't work with modern smart chargers because they will only provide an output if they detect that they are connected a partially-charged battery, but charging one that is not completely dead at a very low current, might show an improvement. However, if you can afford it, probably best to just get a new set: scrap merchants should give you a good price for the old ones.