Why did they change the footstep sounds ?

A lot of people had just the opposite problem as you. Before this season enemy footstep audio was either too quiet, too easily masked, or absent altogether. The changes were meant to address it. Like you I never really experienced the problem myself, but I certainly haven't had any instances of anyone sneaking up on me this season. I haven't tried to sneak up on anyone myself - it would never occur to me to try - so not quite sure why things would have changed for you in that regard. Unless you just switched over to Revenant, who makes no noise by design.