If the deck is within the private yard it is part of the lot and it’s the owners responsibility to maintain it, just like the garden within the private yard is the owners responsibility or the kitchen or bathroom.
Is this an original structure shown on the plans of the complex if so it is structurally the responsibility of the Strata to maintain it, so long as the owner has carried out routine maintenance of the structure.
If it is not an original structure the Strata can refuse to repair it and just tell the owner that it’s not authorized and their is no bylaw so we reserve the right to remove it as a repair option.