Does the ankle rule apply to modern day clothing?
What about wearing pants/trousers that cover the ankle, or is it just the trailing the lower garment with pride is haram?
Keeping the lower garment above the Ankle is applicable even today.
Prophet said "Whatever of the izaar (lower garment) is below the ankles is in the Fire." (al-Bukhaari, no. 5787)
More reports from prophet clearly indicate that the action is to be followed without any opinion or choice even in today's world.
"The Messenger of Allaah took hold of the muscle of my calf (or his calf) and said, ‘This is where the izaar should stop; if you insist, it may be lower, but it should not reach the ankles.’" (Reported by al-Tirmidhi, who said this is a saheeh hasan hadeeth; see Sunan al-Tirmidhi, no. 1783)
Other hadith which speaks of Abu Bakr failing to keep his lower garmnet above the ankles is because his garment kept slipping. Abu Bakr kept his garment above the ankles.
Abu Bakr said to him, "O Messenger of Allaah, my izaar slips down if I do not pay attention to it." He said: "You are not one of those who do it out of pride." (Reported by al-Bukhaari, no. 5784)