For reference this is a 5/8 lock nut with an Allen screw inside. It was so seized up, we let it soak for 3 days with antiseize, tried head and freeze method, and nothing helped. Still ended up with a stripped screw. I’m at a loss at this point and not sure I feel comfortable grinding a notch into it.
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.