Years ago when I interviewed for a Software Engineer position at Google, the person I was matched to eat lunch with between the interviews was from the Search team.
I asked him these exact questions. He said, the last time he checked, quoting a single word to mark it mandatory worked for him and that he definitely would know if it didn't.
I didn't insist much at the time but I knew he didn't know what he was talking about, and it made me lose hope that this feature would ever come back working like it used to.
None that are guarantees, unfortunately. It's maddeningly inconsistent from day to day. But it isn't that rare, either. I only perform 10~20 searches using the quote feature a day, and I hit a case where it's ignored several times a week.
I asked him these exact questions. He said, the last time he checked, quoting a single word to mark it mandatory worked for him and that he definitely would know if it didn't.
I didn't insist much at the time but I knew he didn't know what he was talking about, and it made me lose hope that this feature would ever come back working like it used to.