The reason companies have those fields in there is to benchmark themselves as well. Whilst you may or may not want to divulge that information, you can help your fellow ex-colleagues by putting in a ridiculous amount in there for future salary (believable) and then instead of company name, just put an industry that is similar to the company you are resigning from. If everyone does that and when the bosses review through the data of the exit interview forms, they would realise that they are underpaying their staff.