Because TurboTax believes that you have modified your preciously reported basis, TurboTax won't allow you to delete the explanation statement or the Form 8606. Including the Form 8606 that simply has on line 2 the amount from line 14 of your Form 8606 from 4 years ago and no actual reason to be filing the form won't cause any problems, but not being able to delete the explanation statement means that you'll need to put in some explanation as to why the explanation statement is present. If TurboTax is prompting you to provide an explanation of the change (actually a non-change) to your basis, TurboTax is asking you to enter some text.
The purpose for allowing a change is to accommodate situations where your basis changes due to something like inheriting basis from a deceased spouse, receiving basis with a transfer of an IRA incident to divorce, or rolling over to a traditional IRA after-tax basis from the traditional account in an employer plan. (I don't recall any other circumstances that would require an adjustment to your basis.)
