Beggar's Union Chief Shih For Lung in HSDS never learned more than fifteen of the Hong Lung 18 Palms, and in any case, he was murdered by Sing Kwun.
The Beggar's Union always had the provision of a Skills Transmission/Rewards Elder, whose responsibilities included making sure that the Dog Beating Stick Technique and the Hong Lung 18 Palms would survive even if the Union Chief were killed or went missing.
During the "Lion-Taming Convention" at Shaolin, however, the Beggar's Union's Skills Transmission Elder was murdered by Sing Kwun's men.
Uh-oh.
With both the Union Chief and the Skills Transmission Elder dead, that likely means that both the Dog Beating Stick Technique and the Hong Lung 18 Palms skills were forever lost.
It sure helps explain why the Beggar's Union pretty much disappears from Jin Yong's wulin after HSDS.