It’s important for your boss to frame that differently if it’s truly optional.

They can’t leave it at “do you want to do this project?” They need to follow up with, “It really is optional, I won’t be hurt if you say no.”

They also can’t do it with a big decision first. They have to frame small decisions that way sometimes, to build trust that when they say you have the ability to decline, they mean it. Then, they can ask that way on big decisions.

If a decision isn’t optional, they should phrase it a different way. I use something like - “I need you to do this project - how does that make you feel?” I want them to tell me if it’s a hardship, but I don’t want to imply that I’m leaving the door open to a different outcome necessarily.