I once heard a supposition that before the flood, the whole world was enshrouded in mist so that a large portion of terrestrial water was in the atmosphere. It continues that for the flood to occur, the whole of the mist precipitated (more or less) simultaneously.
The natural conclusion from this supposition is that there could have been no surface rainbows before the flood since direct sunlight did not reach the surface.
NOTE: I do not hold to this particular supposition myself and will not be defending it. Please handle minority opinions respectfully.
I never thought about the poll question explicitly before today but will answer “no.” I’ll offer a caveat that though Noah and his family may have seen rainbows before, this one accompanied God’s promise to not destroy the world in water again. Thus, it was significant.