In the very, very early times of Ren-C, there was no QUOTE_BYTE chewing 8 bits out of the header. So CELL_FLAG_XXX were more plentiful in the remaining header space. Because they were so plentiful, it was picked to use one to say whether a cell was "falsey".
At first, falsey types would be blank, and LOGIC! false. This which expanded to blank, null, and logic false. And LOGIC! could use the flag as its payload.
Today's world with "flexible logic" only has null as "falsey" (though we call it "branch inhibitor" when we want to be technical, since false is a word under that worlview). And LOGIC! is gone as a fundamental type. So even if we had bits to spare, it wouldn't be spent on CELL_FLAG_FALSEY.
In any case, the bootstrap executable has been patched to modern conventions of BLANK! being truthy and such. So this little piece of history is going away. I felt like giving it a goodbye post...
//=//// CELL_FLAG_FALSEY //////////////////////////////////////////////////=//
//
// This flag is used as a quick cache on NULL, BLANK! or LOGIC! false values.
// These are the only three values that return true from the NOT native
// (a.k.a. "conditionally false"). All other types return true from TO-LOGIC
// or its synonym, "DID".
//
// (It's also placed on END cells and TRASH cells, to speed up the Type_Of()
// check for finding illegal types...by only checking falsey types.)
//
// Because of this cached bit, LOGIC! does not need to store any data in its
// payload... its data of being true or false is already covered by this
// header bit.
//
#define CELL_FLAG_FALSEY \
FLAG_LEFT_BIT(18)