Open-source airline route maps from 1999 and 2018 created using Python and BTS data (see corresponding comment for clarifications + GitHub link) [OC] : r/dataisbeautiful
FlightConnections - All flights worldwide on a map!
Airline routes on map infographic Royalty Free Vector Image
interCaribbean | Route Map
Route Map | Our wonderful destinations | PLAY airlines
Visualizing Airline Routes and Hubs – Saumik Narayanan
Flight routes and map projections - Datawrapper Blog
Alliance Air
World Flight Map, World Air Routes Map
Flying Around the World with CartoDB
India Air Routes network Map, Air Routes network Map
Show Off Your Infinite Flight Route Map - Live - Infinite Flight Community