Just because you haven’t paid tax on the SSD benefits doesn’t mean that they are not taxable. Regular taxable earned income (salaries and wages from work) is not taxed if total income does not exceed the available deduction. It’s likely that your SSD benefits have never exceeded your allowable deduction, and that’s why you have never owed tax on them, but that doesn’t mean that they are not (at least partially) subject to tax.
For example, my kid earned $3,000 at a summer job last year, all taxable earned income, but she didn’t pay a cent of tax on it because the standard deduction for a single dependent is up to $6,300 of earned income. So, she had taxable income, but paid no tax.