What is the last date to file a 2019 tax return and not lose a taxpayer refund if a 2019 extension was filed?

No. 
7/17/23

Yes.  If a timely extension was requested, you have until 10/15/23.  

Don't overlook that the IRS knows everything:

https://www.irs.gov/newsroom/time-running-out-to-claim-1-point-5-billion-in-refunds-for-tax-year-2019-taxpayers-face-july-17-deadline

"But the three-year window for 2019 unfiled returns was postponed to July 17, 2023, due to the COVID-19 pandemic emergency. The IRS issued Notice 2023-21 on Feb. 27, 2023, providing legal guidance on claims made by the postponed deadline."

https://www.irs.gov/irb/2023-11_IRB#NOT-2023-21

"Section 6511(a) requires a taxpayer to file a claim for credit or refund within three years from the time the taxpayer’s return was filed or two years from the time the tax was paid, whichever period expires later. Under § 6511(b)(2), the amount of a credit or refund is limited to the amount of tax paid within a specified period immediately preceding the filing of the claim for credit or refund (lookback period). When a taxpayer files a claim within three years of filing the taxpayer’s return, the lookback period is equal to three years plus the period of any extension of time for filing the return. Otherwise, the lookback period is two years. For a calendar-year taxpayer, withheld and estimated income taxes are deemed paid on the due date of the tax return, generally April 15 of each year. See § 6513(b)(1) and (2)."