When a symmetric a/c is flying with a correctly installed sideslip indicator , and zero bank angle , the difference between track and heading is only the wind correction angle ! no reference to sideslip at all ( which is zero) !
Not correct really. Remember back to initial ME-training and engine failure practice. If you are flying
wings level with an engine failure you WILL have a drift (..or side slip if you will) towards the dead engine. The use of rudder to counteract the engine failure will cancel the turning moments but there will be a net force pushing the aircraft slightly sideways through the air. This cannot be detected on the sideslip indicator (which will show zero slip) but the yaw string will tell you the truth.
You can see this during an approach to landing with strong side wind component : if you approach with a sideslip, your sideslip indicator will show , obviously, off-centre,(though track and heading will be the same)
Never done a sideslip/forward slip approach in a C172? Full rudder in one direction and opposite aileron to maintain runway centerline? Heading and track is different.