I just updated to Big Sur 11.4 last night as well and I am also experiencing this exact same problem. The iOS is also the latest version that was updated last night. What is also weird is that we have hooks into a third party messenger which are no longer appearing on pages, thus the reason why I was trying to access the inspector. It works fine on Safari on Mac and other browsers and systems, just not on iOS safari. I wonder if it's related.

@darkguy2008, I think this would get more visibility and traction if you update the title to this post to something like: iOS 14.5/MacOS 11.4 - Latest Updates Are Breaking Safari Web Inspector and Dev Tools