It would be helpful if, when a slip is carried forward, the prior year box values are carried forward as well. The presentation could be similar to the carryforward amounts shown in T1.
I am not sure it would be as helpful as you think.
Why would this be useful?
Payroll from one year to the next bear no resemblance to each other as rate of pay, days worked, sick time off, statutory deductions, etc. all change every year so comparison isn’t even useful.