"Im trying to understand: content from Scirra store that publishers dont port to new store are not available anymore?"
Yes, if sellers do not post their items in the new store, they are not available.
As for the old store, it is part of a website that is being shut down.
If you did purchase items in the old store, they are available for you to download when visiting : construct.net/en/game-assets/downloads
If you have not purchased them yet, and that they have not been uploaded by their sellers/authors in the new store, they are not available in the new (empty by default) store.
As mentioned previously, some "sellers aren't following anymore the store".
We cannot force them and/or do anything about it.