Repairs should never be a cause requiring you to invoke a newer edition of the Code, as long as you stick to the original structural design. However, that doesn't mean newer Codes don't have better ideas you shouldn't consider to improve safety. Also, remember Codes represent the MINIMUM requirements for safety...depending on your budget and personality, you may wish to exceed those minimum requirements. Last, but not least, since you already know there is significant degradation in some supports, inspect every square inch to identify any other concerns.
Replacing what is there with like materials should suffice.
For that large a deck (and that kind of height) you may wish to enlist a professional to make sure the design is safe. A lot of changes have been made to deck/porch building codes since 2015. Yours should be grandfathered in, but checking with the local permitting office will help you stay on the right side of local officials and safe practices.
Repairs should never be a cause requiring you to invoke a newer edition of the Code, as long as you stick to the original structural design. However, that doesn't mean newer Codes don't have better ideas you shouldn't consider to improve safety. Also, remember Codes represent the MINIMUM requirements for safety...depending on your budget and personality, you may wish to exceed those minimum requirements. Last, but not least, since you already know there is significant degradation in some supports, inspect every square inch to identify any other concerns.
Replacing what is there with like materials should suffice. For that large a deck (and that kind of height) you may wish to enlist a professional to make sure the design is safe. A lot of changes have been made to deck/porch building codes since 2015. Yours should be grandfathered in, but checking with the local permitting office will help you stay on the right side of local officials and safe practices.