Tried again on another focal install with identical sources.list where it failed because it tried to download http://ddebs.ubuntu.com/dists/focal/Contents-amd64.gz (the DEBUG lines in the following output come from me inserting a debug print statement in /usr/lib/python3/dist-packages/apport/packaging_impl.py).

apport-retrace -v -s -R -S system ./_usr_bin_gnome-shell-modified.1000.crash 
Get:1 http://ppa.launchpad.net/saiarcot895/chromium-dev/ubuntu focal InRelease [20.8 kB]
Ign http://ddebs.ubuntu.com focal InRelease                                    
Ign http://ddebs.ubuntu.com focal-updates InRelease                            
Get:2 https://mirror.init7.net/ubuntu focal InRelease [255 kB]                 
Get:3 http://ddebs.ubuntu.com focal Release [34.7 kB]                          
Get:4 http://ddebs.ubuntu.com focal-updates Release [34.3 kB]                  
Get:5 http://ppa.launchpad.net/saiarcot895/chromium-dev/ubuntu focal/main amd64 Packages [1604 B]
Get:6 http://ddebs.ubuntu.com focal Release.gpg [819 B]                        
Get:7 http://ddebs.ubuntu.com focal-updates Release.gpg [819 B]                
Get:8 https://mirror.init7.net/ubuntu focal-updates InRelease [79.7 kB]        
Get:9 http://ppa.launchpad.net/saiarcot895/chromium-dev/ubuntu focal/main Translation-en [1084 B]
Get:10 http://ddebs.ubuntu.com focal/main amd64 Packages [523 kB]              
Get:11 https://mirror.init7.net/ubuntu focal-backports InRelease [79.7 kB]     
Get:12 https://mirror.init7.net/ubuntu focal-security InRelease [79.7 kB]      
Get:13 https://mirror.init7.net/ubuntu focal/restricted Sources [7032 B]       
Get:14 https://mirror.init7.net/ubuntu focal/main Sources [838 kB]             
Get:15 http://ddebs.ubuntu.com focal/universe amd64 Packages [4426 kB]         
Get:16 https://mirror.init7.net/ubuntu focal/multiverse Sources [176 kB]       
Get:17 https://mirror.init7.net/ubuntu focal/universe Sources [9792 kB]        
Get:18 http://ddebs.ubuntu.com focal/multiverse amd64 Packages [67.4 kB]       
Get:19 https://mirror.init7.net/ubuntu focal/main amd64 Packages [972 kB]      
Get:20 https://mirror.init7.net/ubuntu focal/main Translation-en [500 kB]      
Get:21 https://mirror.init7.net/ubuntu focal amd64 Contents (deb) [41.3 MB]    
Get:22 https://mirror.init7.net/ubuntu focal/restricted amd64 Packages [21.4 kB]
Get:23 https://mirror.init7.net/ubuntu focal/restricted Translation-en [6136 B]
Get:24 https://mirror.init7.net/ubuntu focal/universe amd64 Packages [8651 kB] 
Get:25 https://mirror.init7.net/ubuntu focal/universe Translation-en [5161 kB] 
Get:26 https://mirror.init7.net/ubuntu focal/multiverse amd64 Packages [145 kB]
Get:27 https://mirror.init7.net/ubuntu focal/multiverse Translation-en [106 kB]
Fetched 73.3 MB in 6s (1072 kB/s)                                              
Fetched 0 B in 0s (0 B/s)                                                      
Extracting downloaded debs...
DEBUG: http://ddebs.ubuntu.com/dists/focal-proposed/Contents-amd64.gz
DEBUG: http://ddebs.ubuntu.com/dists/focal/Contents-amd64.gz
Traceback (most recent call last):
  File "/usr/bin/apport-retrace", line 322, in <module>
    sandbox, cache, outdated_msg = apport.sandboxutils.make_sandbox(
  File "/usr/lib/python3/dist-packages/apport/sandboxutils.py", line 220, in make_sandbox
    pkgs = needed_runtime_packages(report, sandbox_dir, pkgmap_cache_dir, pkg_versions, verbose)
  File "/usr/lib/python3/dist-packages/apport/sandboxutils.py", line 92, in needed_runtime_packages
    pkg = apport.packaging.get_file_package(l, True, pkgmap_cache_dir,
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 490, in get_file_package
    return self._search_contents(file, map_cachedir, release, arch)
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 1325, in _search_contents
    src = urlopen(url)
  File "/usr/lib/python3.8/urllib/request.py", line 222, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.8/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/usr/lib/python3.8/urllib/request.py", line 640, in http_response
    response = self.parent.error(
  File "/usr/lib/python3.8/urllib/request.py", line 569, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.8/urllib/request.py", line 502, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.8/urllib/request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 404: Not Found