Is it safe to assume a wall is not load bearing if the studs are on 24” centers?
Nope...2x4 studs at 24” on center can support roof plus ceiling up to 10’ height.  2x6 studs at 24” on center can support one floor plus roof plus ceiling up to 10’ height without analysis. (See ICC Table R602.3(5).)
Without analysis means an architect or structural engineer could provide calculations to prove that they could be further apart, support more load, or be taller than specified in the Table.