The original post mentions Paya Lebar. It still exists as a military base but is eventually slated for closure.

I'm sure someone has flown between Denver Stapleton and Austin Mueller (Texas). I've been to both but never flew on a flight between both. I'm sure some has flown between Riem (Munich) and Athens (Ellinikon).

Other closed airports include Meigs Field (Chicago) [Springfield, IL service until Mayor Dailey destroyed the airport in the middle of the night], Sde Dov (Tel Aviv), Kai Tak (Hong Kong), Greater Southwest (near DFW), Oneida County Airport (New York) [Continental Connection/CommutAir - up to around 2001], Templehof (Berlin).

So maybe Templehof - Riem? Kai Tak - Athens? Denver - Greater Southwest?

In Houston, there are several heliports no longer in use, such as JPT (Park Ten), JTC (Town and Country), JMA (Marriott Astrodome near the Texas Medical Center). JGP (Greenway Plaza might still be able to reopen). If heliports count, then I can claim an answer! JMA-JPT-IAH and JGP-JTC-IAH.