If I were to make a guess, the "Optional Add-ons" are what would be called Recommends or Suggests dependencies. They are listed by the package maintainer of whichever package you're trying to install, but may or may not be developed by them or be approved by/known to the upstream developers.
And indeed it is so:
$ apt-cache depends rkhunter | grep -Ei 'recommends|suggests'
|Suggests: bsd-mailx
|Suggests: mailutils
|Suggests: heirloom-mailx
Suggests: <mailx>
Suggests: tripwire
Suggests: libdigest-whirlpool-perl
Suggests: liburi-perl
Suggests: libwww-perl
Suggests: powermgmt-base
|Recommends: <default-mta>
Recommends: <mail-transport-agent>
|Recommends: wget
|Recommends: curl
|Recommends: links
|Recommends: elinks
Recommends: lynx
Recommends: iproute
|Recommends: unhide.rb
Recommends: unhide
Recommends: lsof