The sub-slots feature of EAPI 5 was announced as if it was the ultimate solution to the problem of SONAME changes on library upgrades. However, the longer I see it, the more I believe that it is not really a good solution, and that it misses the actual issue targeting somewhere nearby.
The issue is likely well-known by most of the Gentoo users. Every time a library changes its ABI, it changes the SONAME (the filename programs link to) to avoid breaking existing programs. When the package is upgraded, the new version is installed under the new name, and the old one is removed. As a direct result, all applications linking to the old version become broken and need to be rebuilt.
The classic way of handling this is to run the revdep-rebuild tool. It takes a while to scan the system with it but it supposedly finds all broken executables and initiates a rebuild of them. Of course, the system is in broken state until all relevant packages are rebuilt, and sometimes they just fail to build…
As you can guess, this is far from being perfect. That’s why people tried to find a better solution, and a few solutions were actually implemented. I’d like to describe them in a quasi-chronological order.