As far as I'm aware, it always has been. They link to their source code from the footer of their homepage, licensed as GPLv3: https://adblockplus.org/en/source
I was confused with their acceptable ad policy. You looks right, adblock plus seemps open source since ever
Except that their paid whitelisting is anything but open... it's all backdoor deals that are just a profiteering scheme, not about consumer rights or encouraging change.
Plenty of good adtech vendors meet their "standards" but are only allowed through if they pay up. It's all about the money.