While Cassette Beasts NSP + DLC update releases exist in the wild, they carry significant legal and technical risks. The official Switch eShop version runs well post-v1.6.0, and the Pier of the Unknown DLC is reasonably priced for its content. If you must use an NSP for preservation or offline testing, ensure you have a clean dump from a legally purchased copy and apply updates in correct sequence.
For on Nintendo Switch, the most significant expansion is the " Pier of the Unknown Cassette Beasts Switch NSP -DLC Update- -eShop-