I would expect that most foods will contain some form of plastic fragments (plastic fragments from plastic manufacturing/of on packaging or plastic which is knocked off surfaces through processing/handling/packaging). I expect that it would be especially relevant to those which are processed or have plastic packaging/manufacturing plant parts made of plastic.
Most journal papers seem to focus on plastic residues leaching into foods or plastic in the environment rather than plastic fragments within foods. Here is a popular article about this. Maybe plastic in salt is the least of our worries.