I have now uploaded GNOME Shell 46 to Unstable so GNOME Shell extensions incompatible with that extension are now uninstallable. If a compatible version of the extension is not uploaded to Unstable before the the rest of the GNOME Shell 46 transition is ready for migration to Testing, this extension will be removed from Testing.
There is yet another fork of this extension that claims to be compatible with GNOME Shell 46 but I have not tried it yet. No release but you could do a git snapshot or cherrypick commits. https://github.com/mgalgs/gnome-shell-system-monitor-applet On behalf of the Debian GNOME team, Jeremy Bícha
