> I can't imagine a real use case where you'd think this is desirable.
Not S3, but here's a literal real use case: the entry for the Iraqw word /ameeni (woman) in Wiktionary.
https://en.wiktionary.org/wiki//ameeni
If for whatever reason your S3 keys contained English words and their translations separated by a slash, you would have a real problem if one of your scripts were to concatenate woman, / and /ameeni as woman/ameeni instead of woman//ameeni in the English/Iraqw case.