Hi Rainer, diving into each of your questions:
- The open source codebase (Nexus Repository Core) has always been available to the community and will remain as source code on GitHub. Nexus Repository OSS has been sunset and replaced with Nexus Repository Community Edition, from 3.77 onwards all binary bundles are Community Edition.
- The open source codebase (Nexus Repository Core) will continue to be updated. It receives code changes whenever we do Nexus Repository Community Edition/Pro releases. Nexus Repository Core will contain bug fixes, security fixes, and occasional new features.
- The Sonatype Open Source Community Handbook outlines how we accept contributions for all Sonatype Open source projects Contributing to a Project | Sonatype Open Source Community Handbook
- The direct upgrade path is from 3.76 to 3.77 (OSS binary to Community Edition binary). Nexus Repository Core is available to compile, because Core lacks some features included in OSS (and Community Edition) any migration would require some manual steps.