Trip credit is typically - not to say almost exclusively - a feature of non refundable and or non changeable fares bought in the US.

US airlines arguably sell more non changeable fares than anyone else but when you can’t use it they will typically allow « trip credit ». I’m not aware of this being offered in any other market however, so elsewhere the rules are what it says on the tin so that non refundable means exactly that.

And for better or worse, the whole idea of introducing basic/light/promo fares (whatever each airline calls them!) in J and F has been to dissociate cabin comfort from traditional perks such as lounge access, luggage allowance, seat selection and flexibility. QR has been one of the initiators of that trend so many of their business and some of their first fares can be extremely restricted. On OW, AY is another one to watch for on that front. But in short. By and large, « premium classes normally come with some flexibility » unfortunately can’t be assumed any more ever since those fare families are being introduced in J and F by many airlines since they were precisely created to nullify that equation.