I sold a house in 2019 with a remaining mortgage of $918,000 (which started in 2014 at $950,000) and bought another house with a total mortgage of $1,600,000. Is my allowable deduction limited to $750,000 or the earlier amount?
If the former, how do I enter the information in TT?
Because you have a new loan that was started in 2019, the rules in place in 2019 apply. ($750,000 deductibility limit.) You don't get a grandfathered limit. If you enter your 1098 form, Turbotax should ask the questions needed to calculate the deductible amount of your interest.
So anyone who has an "old" mortgage over $750,000 who buys a new house will forfeit the grandfathered loan? Doesn't seem logical since you still have a loan but then I guess it is tax law.
Because there’s nothing to grandfather. The old loan is paid off and closed, and a new loan is created.