Many thanks for the responses. Much appreciated.

Based on the responses and information i have found online i have made the following two lists showing which airlines accept cash and which airlines don't accept cash and covering many airlines worldwide:

Accept Cash:
• Aer Lingus
• Aeroflot
• Air Baltic
• Air Greenland
• Air Malta
• Alitalia
• Atlantic Airways
• Austrian Airlines
• Easyjet
• Egypt Air
• Emirates Airlines
• Etihad Airways
• Eurowings
• Finnair
• Germania
• Gulf Air
• Jet 2
• Kuwait Airways
• LOT Polish Airlines
• Lufthansa
• Oman Air
• Qatar Airways
• Royal Air Maroc
• Royal Jordanian Airlines
• Ryanair
• Saudia
• Scandinavian Airlines
• South African Airways
• Swiss Airlines
• Wizz Air

Do Not Accept Cash:
• Air Canada
• Alaska Airlines
• Allegiant Air
• American Airlines
• British Airways
• Delta Airlines
• Frontier Airlines
• Hawaiian Airlines
• Icelandair
• Jet Blue
• Norwegian Air
• Southwest Airlines
• Spirit Airlines
• Sun Country Airlines
• United Airlines
• Virgin Atlantic
• Westjet
• Wow Air

These lists are not complete so i will keep adding more airlines to them when i find more information.

It's interesting to note that it is mainly USA and Canada based airlines that don't accept cash whilst most (but not all) of the European, Middle Eastern, Asian, African, based airlines do still accept cash. I wonder if there are even any USA and Canada based airlines that actually still accept cash.