The Disclaimer means the passenger has to repay the airline the costs of being flown back if they are denied entry. Its not a thing of the past and some airlines use this system . But it tends to be mainly in the Asia region that this is used . If one is flying from countries like UK , Australia , America etc * its very doubtful that the disclaimer option would be allowed . Most airlines would not want the bother of having to chase you for the money , as wells as possible delays and other complications involved in having to fly back a passenger , so they would prefer to bump you from the flight or make you buy a ticket.
Possibly within the Asia region the staff are more aware that its unlikely that passengers are going to be denied entry to Thailand so they are willing to accept disclaimer in the instances that they ask for an onward ticket in the 1st place.
*( although one poster did say he was allowed to sign a disclaimer form when flying from New Zealand to Bangkok once)
