The tax rules on mortgage interest deductions changed from 2017 to 2018. The limit on the amount of interest has changed under the Tax Cuts & Jobs Act. The interest deduction pre-TCJA has been available to qualified mortgage debt up to $1 million ($500,000 married filling separately).
Through 2025, the TCJA has lowered the amount of qualified mortgage debt to $750,000. For qualified mortgage debt incurred on or before December 15, 2017, the $1 million limit remains in place, thus "grandfathering" existing mortgage debt.