modernize-sources is missing from the apt(8) man page.
I saw this too, but wasn't sure whether it was intentionally excluded as part of the "only mentions the most used commands and options" threshold. On the other hand, `apt --help` does list modernize-sources under a heading of "Most used commands" so there's probably some change needed to the one or the other to achieve consistency.
The main reason is "not duplicate information in multiple places". But this does not apply here because modernize-sources seems to be a command that is specific to apt (apt-get and apt-cache do not know it), and it needs to be documented somewhere.