For the focal verification, I added a secondary test (j2) like I did for bionic, and that is a release upgrade from an updated focal non-server system to groovy, using the base-files package from focal-proposed, thus simulating the release upgrade once this SRU is complete.

Result is correct as well. Details below.

j2) do-release-upgrade from focal to groovy with the focal-proposed base-files package installed, no ubuntu-server installed, and thus motd-news-config disabled:
Starting point:                                                                 
ubuntu@focal-motd-news-split:~$ dpkg -l base-files ubuntu-server motd-news-config | grep ^ii
dpkg-query: no packages found matching motd-news-config                         
ii  base-files     11ubuntu5.2  amd64        Debian base system miscellaneous files
ubuntu@focal-motd-news-split:~$ sudo /etc/update-motd.d/50-motd-news --force    
ubuntu@focal-motd-news-split:~$ echo $?                                         
0                                                                               
ubuntu@focal-motd-news-split:~$ apt-cache policy base-files                     
base-files:                                                                     
  Installed: 11ubuntu5.2                                                        
  Candidate: 11ubuntu5.2                                                        
  Version table:                                                                
 *** 11ubuntu5.2 500                                                            
        500 http://br.archive.ubuntu.com/ubuntu focal-proposed/main amd64 Packages
(...)                                                                           
                                                                                
Just before running do-release-upgrade, I removed focal-proposed from sources.list, to avoid upgrading other packages, not related to this test.
                                                                                
Final state:                                                                    
ubuntu@focal-motd-news-split:~$ dpkg -l base-files ubuntu-server motd-news-config | grep ^ii
dpkg-query: no packages found matching motd-news-config                         
ii  base-files     11ubuntu12   amd64        Debian base system miscellaneous files
ubuntu@focal-motd-news-split:~$ apt-cache policy base-files                     
base-files:                                                                     
  Installed: 11ubuntu12                                                         
  Candidate: 11ubuntu12                                                         
  Version table:                                                                
 *** 11ubuntu12 500                                                             
        500 http://br.archive.ubuntu.com/ubuntu groovy/main amd64 Packages      
                                                                                
motd-new disabled as expected:                                                  
ubuntu@focal-motd-news-split:~$ sudo /etc/update-motd.d/50-motd-news --force    
ubuntu@focal-motd-news-split:~$ echo $?                                         
0