We've not had to apply negative leap seconds yet since leap seconds were introduced in 1972, but that wasn't the point.

The time period of the Earth fluctuates a lot [0] and actually in 2020 it was less than 24 hours, but not a large enough change to warrant a negative leap second. If you go back to the 1940s, we would had needed negative leap seconds if we had leap seconds at all then, and going back 150 years we would have needed multiple negative leap seconds every year for several consecutive years.

What we can say is that on average, it is close enough to 24 hours and the average over hundreds of years is even closer to 24 hours that it's not worth adding these extra seconds as you'd then need to remove them again later on.

[0] https://c.tadst.com/gfx/900x506/graphlength-of-day.png from https://www.timeanddate.com/time/negative-leap-second.html