The LTM4668A is out of stock worldwide and i need to assemble few engineering cards.
One of the outputs is 2.5V/300mA but Vout maximum for LTM4668 is 1.8V according to data sheet...
The LTM4668 and LTM4668A designs only differ in operating frequency. The LTM4668 opearates at 1MHz, the LTM4668A operates at 2.25MHz. Technically since they are same design otherwise, yes the LTM4668 can support output voltages higher than its 1.8V max rating from the datasheet. Note if operating outside the datasheet's specified operating range performance values specified in the datasheet are no longer guaranteed since operated outside of the Vout range spec. covered by test.
The reason the LTM4668 datasheet limits the output to 1.8V max is because of its lower 1MHz operating frequency which gives lowered switching loss. For output voltages higher than the rating (over the rated Vin range) this lower frequency can make the inductor current pk-pk ripple excessively large which modifies current limit behavior and increases loss. For higher Vout support, the LTM4668A's increased operating frequency allows it to cover the higher output voltages while keeping inductor current pk-pk ripple within reasonable range.
In any case, please also make sure other limits are not violated, such as minimum on & off times for your Vin, Vout conditions.