Pinching the nut off could damage the threads so don’t do it. It’s not a big chance but there is a chance. If you can get a file in there and get some movement you can spend hours cutting it that way. It’s the long way but the safe way.

If you have a good file it might only take a few minutes that nut looks like soft metal. But no on the nut splitter. If it was .cheap suspension part yeah but the pinching could crush your threads on the screw.

Can you reach what’s behind the Allen head screw? Maybe get some vice grips in there to prevent that screw from turning and then use a wrench to turn the nut instead.

Other easier than the notch cut might be using a nut splitter on the nut and then replacing the nut instead.

What part is that a toe rod end?