Every proposed legislation I’ve ever seen on this would let the individual tax payer still fill out a Form 1040 and send it in, if they disagree with the IRS prepared numbers.
No, if taxes are actually going to be non citizen-hostile, the IRS needs to get it right the first time. If everyone knows you need to find an appeal to get the real tax rate, you haven't made any progress, just moved the annoyance to the appeal stage.
But it’s not an appeal. It would be the exact same process as if you filed your taxes for the first time.
And for most people the tax bill that arrives would be correct. The parts that wouldn’t be correct would be if there are deductions you’re eligible for, that the IRS may not have any documentation of.
You need to ask why it wasn’t right. It’s not that their computers can’t do basic math - usually it comes down to incorrect data in (e.g. your payroll processor or broker listed something incorrectly) or missing data. In most cases, supplying the complete records will solve that problem.