Only the employment income should be taxed for seniors who return to work after retiring. If they only receive old age security and the guaranteed income supplement, those amounts should not be affected by their employment income. The person's pay should not affect their other sources of retirement income, such as their pension plan. Only employment income after retirement should be taxed.
Such a measure could apply as long as there's a labour shortage, whether for two years, four years, whatever. When the labour market recovers, the measure would end. At that time, if seniors wanted to continue working, they would be like everyone else and would pay taxes on their income.