I can’t say that I was super invested in this, but I think this a good decision because it will be more authentic.
In Vanilla I had to counterspell a flash heal (or other 1.5sec ast) almost as soon as the cast started, otherwise the interrupt wouldn’t go through. This was kind of annoying, but it did make Improved Counterspell a very valuable PvP talent since if I missed the counterspell I would at least get a silence on the target.
I have to admit that I love the responsiveness of retail and that even on a 1.5 sec cast I can still get the interrupt with counterspell even if I don’t get it off until the very end of the cast. This responsiveness though was not in Vanilla and I learned to play around server lag and I think Classic will more authentic because of its inclusion.