XmRemoveProtocolCallback(3) on Linux
The text 1b82336c5d7f, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A VendorShell function that removes a callback from the internal list "XmRemoveProtocol\%Callback" "VendorShell functions" "XmRemoveProtocol\%Callback" "protocols"