This is broken (it NULLs the content if External Store is in use), and I do not believe it's used in the ordinary course of user actions. It is used once by the maintenance script that caused T95580: Flow data missing on Wikimedia production wikis.
I'm not sure we can technically remove it since it extends from ObjectStorage::update(), but we could have it throw an exception.
There is a check that's meant to prevent this, but processExternalStore is not affected. Also, calcUpdates/splitUpdates removes the content fields (because it only updates the columns that changed, and the content didn't), causing it to think it needs to be inserted into External Store.