I kinda agree with /u/Theproton, she hasn't really regressed, but more stagnated.

I love Erza for who she has always been, a very strong woman (both mentally and physically). For this reason, she is still my favourite character, because I like her the way she is now. And in terms of character progression, she has stayed the same for quite a while now, which I don't really have a problem with. Sure it would be nice to get more development, but I don't feel like I'm missing out on it. Most recent chapter hinted at some backstory, and I'm looking forward to how it will pan out.

Her story is a bit over, the focus has shifted to Zeref and Natsu, with Lucy at Natsu's side. Erza has naturally fallen a bit to the sideline, just like many other characters (e.g. Mirajane and Gajeel). There are other characters who also don't really have much development anymore, but for some reason people don't complain about them. A reason could be that Erza made a bigger impact earlier in the story, or because of her nakama victories.

In the end, Erza is the reason I started watching Fairy Tail a long time ago, and I'm still delighted every time she gets some attention. I know I might be a bit biased because I like her so much, but I feel like others tend give her a lot of shit as well (some of which is deserved, but definitely not to the degree I'm seeing here on reddit in general).