seamonkey is not a Manjaro-specific package, it is packaged by Arch. It was flagged out of date in September last year: https://www.archlinux.org/packages/community/x86_64/seamonkey/
It seems that no package maintainer wants to maintain the package or has enough time to maintain it. If no maintainer is found, I guess it will be dropped soon.
arc-gtk-theme is an Arch community package and 20190917 is the latest release. They may switch to jnsh's fork it when the project matures, see FS#65901. If you want to use the fork in the meantime, there is an AUR package, arc-gtk-theme-jnsh-git.
So Liferea had released an update a while ago. The version in the repos is currently 1.12.7, so is there anything blocking it from getting updated? Thanks
teamspeak3 is an Arch community package, not a Manjaro-specific package. It's already updated and is currently in the Manjaro unstable and testing branches. You'll see it in the next stable update.