I don’t buy that. While positive leap seconds can be mitigated at the OS level, negative leap seconds need to be supported at the application level.

A positive leap second gives you a discontinuous function from UTC to TAI. But a negative leap second means a function can’t exist at all, because the same timestamp in UTC now corresponds to two moments in time. If someone gives you a timestamp in UTC you can’t know which second it refers to - you would have to switch to giving timestamps in TAI.