What is the absolute maximum continuous current that XC4000E/XC4000XL and Spartan-XL parts can handle without damaging the parts?
Regardless of Voh, the maximum continuous current should not exceed 20mA per I/O.
The 20 mA limit is due to on-chip metal migration. Aluminum tolerates "only" about 1 mA per micron width of continuous current (thousands of hours). That is a current density of 1 mA per square micron, or 1000 A per square millimeter (about the cross area of a typical lamp cord!).
If this limit is exceeded, the device may still function; however, the effect is accumulative, and the device will be damaged in the long term.
The Spartan-XL devices have a 24mA drive buffer and so, a 24mA maximum current limit.