Thanks to Cédric Luthi, I have returned to a sane world where Command-F brings up a Find panel in Xcode.

Cédric pointed me to his fork of XcodeSFF, in response to my complaints about the user interface in Xcode for doing a "Find". I downloaded the source and followed the instructions:

1. Open XcodeSFF.xcodeproj
2. Build the project, XcodeSFF.xcplugin will be automatically installed into ~/Library/Application Support/Developer/Shared/Xcode/Plug-ins
3. Quit Xcode
4. Relaunch Xcode

There is a known issue that affected me, but again I just followed instructions:

After installing the plugin, you may have to manually set the ⌘F shortcut once in Xcode preferences > Key Bindings > Menu Key Bindings > Edit > Find > Find…

The whole thing only took a few moments. As a side benefit, I have now seen a simple example of an Xcode plugin, which will make it easier to write my own someday if I'm ever so inclined.

P.S. I thought I'd add a few words about what the plug-in does. It finds the "Find…" menu item, which currently exposes the "find banner", and changes its target and action to what they used to be, namely a class called "PBXSimpleFinder" and a method called "showSimpleFind:". If Apple removes PBXSimpleFinder in a future version of Xcode, or modifies it so it no longer implements showSimpleFind:, the plug-in detects this and gracefully becomes a no-op.