“Now”?
This thread is showing me why in the US it is relatively common to fire people with no warning period. I’ve trained my replacements quite a few times, it’s no big deal and a good way to preserve those bridges, but several of our posters evidently feel otherwise.
As you note, severance packages are usually conditional on staying to the end and doing what you are told. Not everyone can tell their employer “screw this,” walk out the door, and have another job the next day. And not everybody has savings. It can take months to find a job between interviews, indecisive hiring managers, management committees, dead-end offers, etc.
And if you refuse to perform your assigned duties, you can be fired for cause and become ineligible for unemployment compensation.
And when potential employers call to check your references, they may not like what they hear. Even if the only question is “Is the employee eligible for reinstatement?” the answer will be “no.”
Unless you don’t need the money or you have some kind of secure situation lined up, it’s a bad idea to burn your bridges behind you. It can come back to haunt you.