Could it be Just Stay There?
Harry is abused through neglect:
The Dursley's had taken up a new tactic to torture him this summer. And boy was it thrilling. It wasn’t physical or anything, simply neglect.
Snape is the one to notice and rescue him in the first chapter.
Crop tops are mentioned in chapter 5:
Then it struck him. They were crop tops.
If it was read on Watpad, plausible match is He Will Be Loved.
The adults who go to retrieve Harry match, with the addition of Moody, and the trigger was not receiving a letter from Harry every three days:
"We've come to check on Harry, Mrs Dursley. We said we would send
someone if we didn't hear from him every three days. I'm sure you
Here's one I downloaded in pdf format
It was probably from rapidshare or similar file sharing site which was linked in a forum
It was being shared as a leaked version of the final Harry Potter book and I think it was even before the name of the final book was released