I think you mean gnome-shell-extension-prefs isn’t installed by default. That package is just a couple of shell scripts and arguably doesn’t need to exist…
The real apps are actually built into gnome-shell itself, so are installed by default in Ubuntu:
-
Command line tool
gnome-extensionsis in binary packagegnome-shell. -
“Extensions” GUI app is in binary package
gnome-shell-commonand you can run it directly:
gjs /usr/share/gnome-shell/org.gnome.Extensions