SVN to GitHub migration has been started today.
SVN has been set to read-only mode and we are currently updating the new GitHub repos.
We are tracking all efforts here: https://trac.osgeo.org/grass/wiki/GitMigration
Much time of the GRASS GIS Community Sprint Berlin 2019 will be dedicated to complete the migration. Further steps will include: some needed source code updates, migration of trac issues, add labels to the issues in GitHub, migration of the trac wiki, extensive aftermath clean-up (Travis CI/CD setting update, email notifications, IRC notifications, cronjobs, fixing links in the website and grasswiki, etc.) and, set-up of mirrors in Gitlab and OSGeo gitea.
Stay tuned for updates!