Per IRS Instructions for Schedule A Itemized Deductions, on page A-7:
Refunds and rebates. If you received a refund or rebate in 2020 of real estate taxes you paid in 2020, reduce your deduction by the amount of the refund or rebate. If you received a refund or rebate in 2020 of real estate taxes you paid in an earlier year, don't reduce your deduction by this amount. Instead, you must include the refund or rebate in income on Schedule 1 (Form 1040), line 8, if you deducted the real estate taxes in the earlier year and the deduction reduced your tax. See Recoveries in Pub. 525 for details on how to figure the amount to include in income.
The "Itemized Deduction Recoveries" section starting on page 23 of IRS Publication 525 Taxable and Nontaxable Income specifically addresses how to determine the amount to include in your income from a recovery of an amount deducted in an earlier year as an itemized deduction.
Worksheet 2. Recoveries of Itemized Deductions on page 26 of IRS Publication 525 will help calculate the part of a recovery amount to include in your income in the current year.
If you find you need to enter a description and an amount for Line 8 of Schedule 1 (Form 1040) Additional Income and Adjustments to Income:
When you print your return, the text "See Attached" will print to the left of Line 8 on Schedule 1 (Form 1040). An attached sheet will print with the return titled "Other Income - Supporting Details for Schedule 1 (Form 1040), Line 8" which will print both the description and amount that was entered.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.