This is the wrong place to draw the line. The question appears to be advocating that any question which has a business or working component be declared off topic.
If the business world is the focus of the question then it belongs on the workplace. If the focus of the question is the academic environment then it belongs on academia.
Examples:
There is a person that I work with that I would like to ask out. We
are in different department but I am a manager and they are not. Is
it inappropraite for me to attempt to make a romantic advance on her
out side of work?
This question is clearly about the business environment and acting professional, it belongs on The Workplace. As opposed to...
There is a person that I work with that I would like to ask out. We
are in different department but I am a manager and they are not. My concern
here is they may think my advance implies some quid pro quo offer and
I would like to be able to make it clear that it does not, but also
not shoot my self in the foot and be awkward. How can I approach them
in a way that makes it clear this is a person conversation with no
implied or direct inference to any work?
This question has a business relationship context but it is about the interpersonal relationship. This question could probably be appropriate on either the workplace or here, but this question is clearly in the wheelhouse of IPS, and since it is about the personal relationship on topic.
In your examples:
How can I politely tell someone I won't do my job for free? - This is a question for the workplace. It is how do i maintain a proper business relationship, not personal relationship.
Can we use abbreviations in professional communication? - This is a question is an example of overlap. It would work on either site - It is general enough to be applied in non business related activities, but also has enough business context that it would be appropriate on the Workplace.
How do I tell certain customers they're not my number one priority? - Is not really about interpersonal communication its about what the right business decision is to make.