The webRequestBlocking is not supported in Safari. One of the use cases in extensions is the ability to remove the x-frame-options header from requests so some pages can be iframed within the extension popup. See also: https://feedbackassistant.apple.com/feedback/9612497 https://feedbackassistant.apple.com/feedback/7848317 https://developer.apple.com/forums/thread/650840
Thank you for the report. This will continue to be tracked by Apple internally as a safari issue, not a WebKit one. rdar://57367706
We are tracking bugs in Bugzilla for Web Extensions now as we move extensions support from Safari to WebKit.
seeAlso https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/BlockingResponse