As disablement benefit has always been treated as income for tax purposes the Deputy is probably referring to disability benefit, which has been reckonable as income for tax purposes since 1993.
It is a general principle of taxation that, as far as possible, income from all sources should be subject to taxation. In line with this principle the majority of social welfare payments are therefore reckonable as income for tax purposes. Treating such benefits as income for tax purposes is essentially a matter of equity. Long-term social welfare payments have been taxable for many years. Taxation was extended to disability benefit in 1993.