Having the same problem for iPhone 5 / iOS 10.3.4 / macOS 10.15.7 / Safari Desktop 14.0. What is interesting is that when I go to iOS Settings → Safari → Advanced and toggle ‘Web Inspector’ off and on again, the Develop → iPhone menu in Safari Desktop briefly changes to ‘No Inspectable Applications’ and then goes back to ‘Enable Web Inspector on Device’. All this looks like a connectivity / broken contract bug. All this is quite a shame as it leaves no options to debug a broken website on a legacy device with iOS 10 🤔