Hi
The repo repo.boundlessgeo.com is going to be shut down and repo.osgeo.org will act as a replacement. For the branches master and 3.10.x the following pull request is applied: https://github.com/geonetwork/core-geonetwork/pull/4595
Custom forks of the project will be required to apply it. Also previous branches, initially will not be updated.
A workaround without requiring the apply the changes from the previous pull request is to update the local settings.xml. You can find more details in https://docs.geotools.org/latest/userguide/welcome/upgrade.html#alternative-mirror-retired-repo-boundelssgeo-com