Why do you need the "Webpage Contents" permission?
All versions of AdBlock (like all ad blockers in general) try to hide ads they can't prevent from being downloaded. To hide an ad, we tweak the web page code that displays it. In other words, we change the page so that it doesn't include downloaded ads.
Most versions of AdBlock also tweak the page code to display our whitelist wizard on request. In other words, we change the page to include an element that wasn't originally present. The wizard allows users to interact with AdBlock to fine-tune the exception rule it creates. We added a whitelist wizard to our Mac app in the version 1.15.0 release. The part of the Mac app that displays the wizard is the AdBlock Icon extension, which opens the AdBlock menu in Safari. And that is why AdBlock Icon now needs "Webpage Contents" permission.