Unless they are happy with degraded performance or needing to switch to developer mode often, users may have to chose between ad-blocking and custom scripts.

Just going by the name, chrome.declarativeNetRequest.onRuleMatchedDebug, I'd expect that to be something that has to explicitly be used. Enabling developer mode would make it available which they say might reduce the performance "depending on how it's used", but would a production release of an ad-blocker extension be using it?