I am running openSUSE Tumbleweed 20200123. I have added the repository for yeacreader (download.opensuse.org/repositories/home:...openSUSE_Tumbleweed/) and installed it. It says this is verion 9.6.2.1909283-7.5, which as far as I can tell should support RAR5 according to the change log on the homgepage. The libunarr1 dependency is also installed. .rar files that are not RAR5 open fine, but RAR5 does not. The error I get when opening a RAR5 file is "Format not supported." Am I doing something wrong?

The libunarr backend does not support RAR5 yet. I am working on adding support and have made some progress, but it still is a work in progress.

As of 9.6.2 RAR5 is only supported by the p7zip backend which needs to be enabled at compile time. p7zip is currently abandoned by its upstream, has license compatibility issues and to build with this backend you need to include extra source files. The upstream code also has known security issues (these are patched for the official packages shipped by distributions), so all in all it is a dead end.
This situation is a bit different for the Windows builds as 7zip (without the "p") is still in active development.