> On Arch, packages just kinda vanish from the repos it seems like.

Arch isn't designed to hold your hand, it encourages users to do their own research and due diligence, it expects responsibility from their users.

Arch Linux is nothing more than pacman and makepkg, both are designed to give you bare-bones packaging capabilities, how you use those tools and manage your system is up to you.

pacman has the -Qm option which will show you packages that are no longer in the database, so you very much can do the same, i.e. plan ahead.

A major difference of portage (in contrast of pacman) is that portage has a lot more abstractions (USE flags, EAPI, news, etc) but those come at the cost of complexity.