Is this SLA battery still usable?
A battery has to pass several tests to be 'not dead'.

Settling to an open circuit voltage of 6.3(ish) volts, and staying there, is a good first sign.

You should also check the capacity. These batteries are specified at the 20hr rate. That doesn't mean you can't discharge them faster, but that you'll only get the specified capacity at that rate. 12Ah/20 is 600mA. After a full charge, discharge them down to 5.5v. You would expect the capacity to be a substantial fraction of 12Ah.