The red underwear would look out of place if they tried to bring it back now.
It's not underwear. It's a pair of trunks, like swimwear. It's the sort of thing that circus acrobats and strongmen in the 1930s wore over their tights for modesty.
And I don't think it would look out of place for Superman to wear any more than the cape or the boots or the big S on his chest. It's just part of Superman's costume. And it works better visually than just an unbroken expanse of blue.