They are still short enough not to type much when using them, but long enough to be obvious and not to freak out linters that require a minimum identifier name length.
Hmm, idx has actually tripped me up before when looking at someone else's code. In my head I read this as "id x", and took me a while to figure out it was the index. I feel like if there is a reason to shorten variables, i would be more descriptive as it is more common. However, since you're already making it slightly longer, why not just name it index?