Assuming you have a Switch with custom firmware (Atmosphere, SX OS, or ReiNX) and SigPatches enabled:
To have "all DLC" for Sonic Forces on Switch, you need to collect six distinct downloadable items. Some were included with special editions, while others were free promotional downloads. sonic forces switch nsp update all dlc exclusive
The digital wind howled through the hidden directories of the "Blue Blur" underground forum. A user known only as had just posted a magnet link that set the servers on fire: Sonic_Forces_NSW_AIO_ULTIMATE_V1.05_ALL_DLC_UNLOCKED.nsp . Assuming you have a Switch with custom firmware