Relief u/s 89(1) available under New Tax Regime ?

1. Yes

 2. Correct...

For tax calculator on arrears ....

Refer:: https://cleartaxindia.com/how-to-file-10e-in-the-new-income-tax-portal-with-auto-calculate-income-tax-arrears-relief-calculator-u-s-891-with-form-10e-from-the-f-y-2000-01-to-f-y-2023-24/

Section 89(1) relief is available under both the old and new tax regimes. The critical step is that you must file Form 10E on the income tax portal before you submit your ITR. If you file the ITR first without Form 10E, the department disallows the claim automatically and you get a demand notice. The calculation looks at your tax liability with and without the arrear in the relevant past years and gives you credit for the excess.

Full guide on how to calculate and file Form 10E here: https://taxgarden.in/blog/section-89-tax-relief-salary-arrears-form-10e