Ah, it appears to be the RetroPixelMax script.
Viewing post in Variety of retro shaders for Unity, get them while they're hot
Yeah the names are old and need to be updated, RetroPixelMax is the one that lets you define a fixed palette. Also make sure you put the effects in this order:
- Dither
- RetroSize
- RetroPixelMax
if you put dither after RetroSize, the dithering will not be downsampled and you'll break the resolution restriction of the jam