I need the opposite of `..<index`, why is there no `index>..`?

As @Lantua explained up-thread

And expanding on it a bit, this is because of the limit of character combinations allowed in an operator for it to be parsed unambiguously.

If you need left-open right-closed intervals, I have an incomplete library Interval on GitHub. However, it has some significant disadvantages compared to RangeExpression types.