I am not sure from what date the slipway was decommissioned. The earliest picture I managed to find of it was taken around 1912.
Some entrepreneur wanted to use pylonns to build a pier back in the 1980's and applied for permission to do so. The plans were firmly squashed by a bureaucrat in the old South African Railways and Harbours, claiming it was their property because it was below the high water mark. Needless to say the project never proceeded.
The pylons have now become a roosting place for the local cormorants, which I suppose means they still serve a useful purpose