[nullc]

The often made and only technically true claim that posix/unix time "ignores leap seconds" is actually a major source of leapsecond handling bugs, because it makes people think that posix time is TAI (with some offset). Instead posix time is UTC with some seconds that have unusual lengths (0 or two SI seconds, as you note).

When you want to know the number of SI seconds between two dates in either posix or UTC you need an accurate table of past leapseconds. If you want to compute TAI from posix or UTC you need an accurate table of leap seconds that your UTC clock has applied (which may not be the same as the table of leap seconds, since you might have missed the most recent one!).