It's still trivial to detect Incognito and other private browsing modes. Our web installer is an entirely unintentional implementation of Incognito mode detection.
grapheneos.org/install/web
It doesn't work in Incognito mode and throws an error due to memory-based storage limits.
Conversation
Those storage limits could be detected locally without downloading a bunch of data. It seems like the intention is supposed to be that Incognito mode can't be trivially detected, but we can't even figure out a way to work around this and not throw an error in Incognito mode.