Thai Airways has myriad inflight products. They’re known for aircraft swaps at the last minute. Although in recent years they’ve tried to rationalize their fleet as part of an effort to stem losses, with the Thai national carrier you never quite know what you’re going to get.
One of the things that’s different in the fleet is that the reverse herringbone seat in their Boeing 787-9s. And these seats have seat belts with airbags. They can’t accommodate waists larger than 56 inches, and you cannot add a seat belt extender to them or else the airbag would be misaligned. This size limit also means parents with lap infants can’t sit in these seats.
So Thai Airways has banned passengers of a large size and parents with lap infants from the new business class on their Boeing 787-9s. They only have two of these planes and aren’t getting more. So much for rationalizing the fleet, and don’t expect clear communication on these limits from Thai either.
I suspect that 56 inch waists, or 56 inches including a lap infant, doesn’t come up too often. And that’s why other airlines with airbags in their seat belt assemblies haven’t been flagging this issue. However Thai has gotten explicit with the restriction.