And the answer is... bugs.
Bug #1: There's some path by which upgrading to Logic 10.5 from 10.4 disables Define Brush Pattern. The fix is to go to Advanced Tools and turn on all the Additional Options, although I suspect that "MIDI" is the culprit. This enables the context menu item as documented. Note that having the Additional Options on is not required, it's just that turning them on resets whatever glitch is in the configuration.
Bug #2: CTRL+SHIFT+B doesn't work by default, contrary to the documentation (https://support.apple.com/guide/logicpro/piano-roll-lgcpfa34642b/mac). Not only is the keyboard shortcut not mapped unless Advanced Options | MIDI is enabled, but none of the keyboard commands in the entire Piano Roll section are able to be mapped. This is a bug, since enabling an advanced feature that "Allows signal flow control and real-time processing of MIDI input and output streams in the Environment" has nothing to do with whether I should be able to create a shortcut to execute behavior that's available by default. Again, the documentation says this keyboard is by default, and mentions nothing about "MIDI" advanced configuration.
Bug #3: With Advanced Options | MIDI off, the context menu in the piano roll area nevertheless says that CTR+SHIFT+B is the shortcut for Define Pattern Brush, but that shortcut doesn't work.
And for what it's worth, the documentation (https://support.apple.com/guide/logicpro/add-notes-lgcpa904cb3a/mac) says that to define a brush pattern, you need to "Control-click the selected notes with the Brush tool". You can actually click anywhere in the piano roll area.