In 2020, Turbotax prompted me to include depreciation on my home, which is rented since mid-2018. Why didn't it do this in 2018 or 2019?

Rental depreciation is the joy of a landlord and the program has always had it.  You must have done something with an asset that triggered a depreciation question.

You will want to go back and amend your 2018 and 2019 returns to include the depreciation. The IRS will send you the refund plus interest.

To file:

• Save a copy of the original return before doing an amended return. See: 
  • how to file an amended return.
  • I need to amend my state return.
  • Can I e-file my amended state return?
  • Can I e-file my 1040X to amend my return?
• Use separate forms if amending more than one tax year.
  • Each year on its own form
  • Each year in its own envelope
• Track amended return status online. Taxpayers can track the status of their amended tax return in English and Spanish using Where’s My Amended Return?  Amended returns take up to 16 weeks to process and up to three weeks from the date of mailing to show up in the system, longer with COVID.

You are required to depreciate rental property, every year that property is "in service" as a rental asset. Work through the asset again in the Asset/Depreciation section. One of the screens will show a box for "prior depreciation". If the property was classified as a rental in 2018 and 2019, then there will be an amount in that box greater than zero. If this is not the case, then let us know. You may have a major tax issue.