What SQ was/is trying to restrict is long-haul premium business class travel (ie full flat premium beds). These are traditionally found on the 77W and A380 (although have been extended lately to other aircraft). As these aircraft also operate some flights to Asia (often as part of tag flights), those routes were also restricted.

Regional flights have generally been better for availability because they fall outside the above category. Sometimes a flight is 'regional' but will happen to be operated by a long-hauler some of the time. In those cases, you might still be able to book the flight and enjoy the premium product.