Stop Enabling Sex Traffickers Act of 2017 (SESTA) S. 1693.  
Similar Bill in the House (H. R. 1865)  
“Allow States and Victims to Fight Online Sex Trafficking Act of 2017" aka FOSTA

I’m only speculating but if I had to guess why the change to BP’s format I would say:
In accordance with the new 2017 Law that was passed saying that victim’s "providers' can now sue adult websites.

I would guess BP is trying to take some precautions by removing some of this type of content. While again I’m only guessing that they know this can and will lead to being sued because some other site is using the information pulled from their site.

They’re probably trying to stop any outside activity (Web crawling) of they’re site

To web-craw or web-harvest you’re basically doing a text search.  Now that the text part has been removed, all that is left is

Pictures
Controlled Phone Number
Hyperlinks

“Pictures”
Now, this day and age we have image search, and to get into that is too much logic. something about a couple of guys learned to come up with an equation to make the image into numbers.
But if you don't know those numbers you really can’t do a search by typing it in the search back, like how a web crawler would have to.

“Hyperlinks”
Hyperlinks are kind of potholes. Most of them are small, but then you hit that big one. the program your running needs to be ready for anything. You never know what’s going to happen when you click on a link. As long as it’s not a link back to BP it isn’t their problem.

“Controlled Phone Number”
The new controlled phone number format. I can only guess this was for indexing and in their database and simplify the logic they had to use to decrypt some of those numbers. It may have been because over 72% of the numbers being listed on BP were no either deactivated right after the add was posted or very shortly after. I would assume that also lead to their desertion to only allow connected numbers giving more attention to the local ladies, but I don't see that lasting. Must have more to do with cellular vs VOIP

I'd go into data-extraction and the difference between cellular data and VOIP but I think, for now, that help us accept change