Why DDL are not supported on intermediate database in a cascading replication environment, rather, to put it aptly, why it is not recommended.
It is not that the DDL will not replicate to the final target if executed on the intermediate Shareplex instance.
If you do DDL on A, it gets applied to B as well as C. However, if you do it manually on B, it still propagates to C but A does not receive the DDL changes. So there will be inconsistency between A on one hand and B and C on the other, as far as the object in question is considered. This is the reason it is not supported to do DDL on B as such inconsistencies can result in Post errors on C. For example, if you add a column col_1 on B and now both B and C have that column. If you subsequently added the same column col_1 to the object on A, then Post on B will stop due to DDL error ORA-01430 column to be added already exists. This is the reason it is not supported.
