No, software installed by SCCM-client doesn't suddenly disappear from client computers when SCCM Primary Site is uninstalled. You'll need to advertise the removal of program to uninstall software from targeted computers.
If you want to completely remove all SCCM-related programs and not reattach all SCCM-clients to other sites, then remove first all SCCM-clients by entering the following from the command prompt on the client computer:
If you want to uninstall the SCCM-client from many computers, then you may create a package using Configuration Manager software distribution that will distribute above command to targeted computers.
Use the log file CCMSetup.log in the
%windir%\system32\ccmsetup folder on the client computer to verify that the SCCM-client has been removed succesfully.
To uninstall a Configuration Manager primary site, you can use the Configuration Manager Setup Wizard. You can either start the wizard from the installation source files or you can use the Add or Remove Programs applet in the Control Panel by selecting the installation to uninstall.
If you want to reassign SCCM-clients to another site, then to avoid orphaning clients or child sites of the site to be uninstalled, you'll need to ensure that any child sites have been reattached to other site and any assigned clients have been reconfigured to be managed by a different primary site.