You can do this without modifying your system.
Use something like Pacifier to extract the AirPort Utility.app and Apple80211.framework from the installer.
Then, run these two commands in Terminal whenever you want to use it (substituting the correct paths obviously):
$ export DYLD_INSERT_LIBRARIES=<path-to-older-Apple80211.framework/Versions/A/Apple80211>
$ /path-to/AirPort\ Utility\ 5.6.1.app/Contents/MacOS/AirPort\ Utility
I keep that older framework and older app in the same folder, save the following AppleScript as an app (it expects to be in the same folder too), and just double-click it to launch:
tell application "Finder"
set base_folder to (container of (path to me) as alias)
set base_path to POSIX path of base_folder
set library_path to base_path & "Apple80211.framework/Versions/A/Apple80211"
set library_exists to exists (library_path as POSIX file)
if library_exists then
do shell script "export DYLD_INSERT_LIBRARIES=" & quoted form of library_path & "
" & quoted form of (base_path & "AirPort Utility 5.6.1.app/Contents/MacOS/AirPort Utility")
display alert "Apple80211.framework could not be found" message "The library was not found in " & base_path & "."
This works all the way up to Mojave.