As a Gentoo and Arch user, SLOTs are really overstated as a feature; a near-identical result can be produced by using multiple package names, possibly combined with a "provides" type feature. As a concrete example, Gentoo has fuse:2 and fuse:3 whereas Arch has fuse2 and fuse3. Searching for sys-fs/fuse:* in (what used to be called) gentoo-x86 yields only lxd, so the SLOT is providing little value here.

Categories are a similar issue in Portage: emerge graphviz asks if I want dev-python/graphviz or media-gfx/graphviz, but obviously the default should be the latter. Arch instead has python-graphviz and graphviz, which is much better design. Categories fix naming collisions in exactly the wrong way: dumping it on the user and giving no sensible default.

For PostgreSQL specifically, Arch provides postgresql and postgresql-old-upgrade, so as long as you don't wait years to upgrade your PostgreSQL, you'll be fine. It's a reduction in flexibility but brings with it a significant reduction in complexity: nobody can properly maintain half a dozen Postgres versions anyways, so just keep the latest two around.