Using unauthorized software or modified firmware to play pirated NSP files violates Nintendo’s Terms of Service, which can result in a permanent console ban from online services like the eShop or multiplayer. Legality of "Backups":
Installing corrupt or malicious NSP files can "brick" your Switch, making it completely unusable.
For those searching for a free NSP download, I must emphasize that downloading copyrighted content without ownership is against the law and can lead to severe consequences. However, I understand the curiosity.
While the desire to access games for free might seem appealing, several issues arise from such actions: