Airports that I have walked to/from successfully:
Halmstad, Sweden (HAD) - Easy walk into town, though using the bicycle lanes
Qingdao, China (TAO) - After crossing through the car park, could walk out and anywhere on the sidewalk
San Diego (SAN) - Walked to and from downtown, though when first trying to exit I tried to leave first crossing a surface car park but was blocked from accessing the road by a chainlink fence which I climbed, scraping myself. On the way back, I found the right way.
Changhai Island, China (CNI) - Tiny airport, could just walk along small roads to the town. But not made for pedestrians.
Hamilton Island, Australia (HTI) - Walked along the quiet road into the terminal
Gibraltar (GIB) - Walked across the runway
and into the terminal from the city centre
Mexico City (MEX) - Not pedestrian friendly, but followed many people out of the terminal walking on the exit road until a main street with a sidewalk.
Saipan (SPN) - Not pedestrian friendly, walked a very long way from my beachside hotel along busy roads with no sidewalk, eventually reached it. Not recommended.
Nantucket (ACK) - Easy but slightly long walk along trails and sidewalk to the town.
Whitehorse, Yukon (YXY) - Also not made for pedestrian entry, but quiet enough to walk in along the road.
Barrow, Alaska (BRW) - The airport is right in the town, so easy to walk in and out and all around the small city.
Ponta Delgada, Azores (PDL) - Longish walk but sidewalk all the way to the city centre, though the streets become very narrow once in town.
Myrtle Beach (MYR) - Not made for pedestrians, but could walk in the grass along the road to enter. Not recommended.
Manistee, Michigan (MBL) - Walked along the highway from the airport to town. Scary at night with fast traffic. Not recommended.
Los Angeles (LAX) - As others have mentioned, walked from Terminal 1 to the In-N-Out. Also walked to the east to a hotel before.
Manila (MNL) - Have walked to the casino, hotels, aviation museum, restaurants from the airport by crossing the parking and onto roads. Just crossing the street outside Terminal 4 can get you into a shopping centre.
Block Island (BIT) - Could walk along the road right out of the airport. It was winter so very few cars on the roads.
Westerly, Rhode Island (WST) - Sidewalks from the terminal to anywhere in town - walked to the train station.
Marble Canyon, Arizona (MYH) - Walked on the shoulder of the highway from the airport to the visitor center of the Navajo Bridge
Imperial, California (IPL) - Walked on the shoulder of the highway from the terminal to get Mexican food at a nearby shopping center.
St. Pierre (FSP) - Walked along the side of the road to and from the airport several times during my stay in St-Pierre et Miquelon.
Rochester, UK (RCS) - I ended up making it to the airport from the railway station, but there was no pavement in several places and it did not feel safe.
Guernsey (GCI) - Had a great walk from the airport to the seacliffs along country lanes.
Isle of Man (IOM) - Walked from the airport to Castletown and back - I believe there was pavement.
Duxford (QFO) - Walked from a nearby railway station to the airport using a scenic dirt trail.
Le Bourget (LBG) - Easy to walk into any of the FBOs from the surrounding city area.
Bern (BRN) - Walked from the airport into the nearby Belp town to a hotel using cycle paths/pavement.
Warsaw (WAW) - Walked into the airport from nearby town area on sidewalk.
Wels, Austria (LOLW) - Walked out of the airport and along residential streets into the town centre.
San Carlos (SQL) - Was able to walk out of the airport and into the commercial areas on sidewalks.
Blackpool (BLK) - Adjacent to a residential area, a long but easy walk on sidewalks to the centre of town.
Tiree (TRE) - Walked out of the airport and down the middle of the road and all over the island. Thankfully, very little traffic.
Donegal (CFN) - Walked out of the airport and down the road to the nearby beach.
Charlotte (CLT) - After visiting the Carolina Aviation Museum, I wanted to go back to the terminal for my flight and thought I'd walk. First I found myself walking on the airport perimeter road, a police car passed me and took no notice. Then, I walked down a slope and was walking on the shoulder of the interstate. At an overpass, I went back up the slope and along the side of that road and found myself at an employee parking lot. I boarded an employee shuttle that took me back to the terminal.
Las Vegas (LAS) - I wanted to go to a nearby airport hotel but it was too late for the shuttle to operate, so I tried to walk there from the airport. I tried to cross several parking lots but was always met by high fences. Eventually I returned and took a rip-off taxi ride to the hotel.
Stansted (STN) - Wanted to walk to a nearby airport hotel, but it seemed the only way to leave the airport was to get onto a road with lots of fast traffic. Took an Uber in the end.
Newquay (NQY) - This is from the old Classic Air Force museum rather than the terminal. I wanted to walk back to town from the museum, but the road is narrow with steep mounds on either side. I had to run up the mound each time cars would come. Eventually someone stopped who was worried for my safety and gave me a ride the rest of the way.
Atlantic City (ACY) - Thought of walking out of the airport to see what was around, but seemed I would have to walk on the road to do so.
Newport News (PHF) - No way out without walking on the road itself, so I ended up taking a taxi which drove in the wrong direction and pretended not to understand me and then later used my credit card number for fraudulent purchases.