Question about QSBs

The new 4.2 version of Qubes was released in December. Are the QSBs prior to the release datum also still relevant or can I assume that these issues are already fixed in the new version?

Every Qubes ISO contains all security patches that were available at the time the developers started the process of creating and releasing that ISO, but that process takes time, so it’s always at least a little earlier than the actual release date. Regardless, you should always update your system after installing from a new Qubes ISO in order to ensure you have all the latest security and non-security updates.