The SObjectizer-5.8.1 introduces a couple of new features and fixes several issues.
The so5extra-1.6.1 contains adaptation to SObjectizer-5.8.1. Please note that due to changes inside the SObjectizer Core the previous versions of so5extra may not work with v.5.8.1 properly. If you upgrade to SO-5.8.1 you have to upgrade to so5extra-1.6.1 too.
PS. There are pending PRs for vcpkg and conan, we hope they will be approved soon.