This has been interesting. For reference, see also Where does 'exists base package "X" of debianpackage' look?
But basically Debian/Ubuntu records all packages the system has even seen, even if they have been uninstalled. So if the package was never installed, the relevance will return False. Once it is installed, it will return True and after its uninstalled it may still return True. So vendors may have their packages totally erase themselves from the apt database, but you can’t depend on that. So the relevance I really wanted is.
Q: exists packages “splunkforwarder” whose(currently installed of it) of debianpackages
A: False
T: 390128
and not
Q: exists packages “splunkforwarder” of debianpackages
A: True
T: 438164