As a tool developer I want to only get data of the statement when it has changed so that I do not need to process data I already have. This should work the same way for both GET /entities/items/{item_id}/statements/{statement_id} and GET /statements/{statement_id}.
If-None-Match and If-Modified-Since request headers should be used for consistency with conditional requests to /entities/items/{item_id}
Use item's revision ID and item's last modification date as values to check against the user-specified ETag or modification date. Add a note to the OpenAPI document for these two routes that the ETag refers to the Item revision id. It should behave the same way as T302060.