The cyrus-sasl package contains a sample directory with examples in C that demonstrate how to build against libsasldb. These examples are difficult to build outside the cyrus-sasl tree and offer no value for the installed package in Gentoo, which is why they are no longer packaged. If you intend to develop e.g. plugins, please see the upstream development repository at https://github.com/cyrusimap/cyrus-sasl.