I don't think a negative leap second would cause much of an issue at all. The reason a positive leap second was such a problem was because it add an "invalid entry", ie. 23:59:60.
But a negative leap second would just be skipping a valid entry, which happens all the time if you're using NTP to set your clock.
Sure, date math libraries would need to updated, but they already got updated for the positive leap second to have tables they can look up to find out which seconds to add, so adding an entry for a skip isn't much different.
Most of the havoc of the positive leap second was in time series data having that invalid entry. That won't happen with a negative leap second. There just won't be a value there.
But a negative leap second would just be skipping a valid entry, which happens all the time if you're using NTP to set your clock.
Sure, date math libraries would need to updated, but they already got updated for the positive leap second to have tables they can look up to find out which seconds to add, so adding an entry for a skip isn't much different.
Most of the havoc of the positive leap second was in time series data having that invalid entry. That won't happen with a negative leap second. There just won't be a value there.