This is the core of the keyword. “All DLC new” means every piece of downloadable content ever issued for the Switch version. Unlike other platforms, Switch received most (but not all) of the PS4/PC DLC.
Here is the definitive list of all DLC available for Sonic Forces on Nintendo Switch: sonic forces switch nsp update all dlc new
Disclaimer: This guide is for educational purposes. Installing NSPs requires a hacked Switch and custom firmware. Only use dumps of games you legally own. This is the core of the keyword
Release Date: November 7, 2017 (worldwide)
Publisher: SEGA
Developer: Sonic Team
File Size (base game): ~6.5 GB sonic forces switch nsp update all dlc new