The Boundless open source project has been archived and is no longer maintained.

The osgeo and boundlessgeo maven repositories used by the GeoTools projects have been migrated to a new repository manager setup by the Open Source Geospatial Foundation.

We tried adding a redirect to continue to allow builds to run with out modification ... but maven is not setup to follow redirects safely.

Reference:

PlanetLabs notification (Twitter)
Announcing repo.osgeo.org for maven, docker, npm, … (OSGeo)
repo.osgeo.org (OSGeo)
SAC:Repo (OSGeo)

download.osgeo.org/webdav/geotools releases

The main geotools release repository has been moved to:

<repositories>
  <repository>
    <id>osgeo</id>
    <name>OSGeo Release Repository</name>
    <url>https://repo.osgeo.org/repository/release/</url>
    <snapshots><enabled>false</enabled></snapshots>
    <releases><enabled>true</enabled></releases>
  </repository>
</repositories>

To fix your build please update your project pom.xml as shown above, or define the following ~/.m2/settings.xml mirrors:

<mirrors>
  <mirror>
    <id>osgeo-release</id>
    <name>OSGeo Repository</name>
    <url>https://repo.osgeo.org/repository/release/</url>
    <mirrorOf>osgeo</mirrorOf>
  </mirror>
  <mirror>
    <id>geoserver-releases</id>
    <name>Boundless Repository</name>
    <url>https://repo.osgeo.org/repository/Geoserver-releases/</url>
    <mirrorOf>boundless</mirrorOf>
  </mirror>
</mirrors>

References:

Change to repo.osgeo.org for GeoTools releases (User Guide)
Alternative: Mirror retired repo.boundlessgeo.com (User Guide)

repo.boundlessgeo.com snapshots

The boundless repository was used by our build server to publish snapshots, it is moved to:

<repository>
  <id>osgeo-snapshot</id>
  <name>OSGeo Snapshot Repository</name>
  <url>https://repo.osgeo.org/repository/snapshot/</url>
  <snapshots><enabled>true</enabled></snapshots>
  <releases><enabled>false</enabled></releases>
</repository>

Please note that this is populated by our build server for the active master, stable and maintenance branches. Snapshots for historical builds are not being generated! If you find a project built using a snapshot from an archived version of GeoTools please update to the appropriate release.

Reference:

Change to repo.osgeo.org for GeoTools snapshots (User Guide)
Upgrading projects using historical GeoTools snapshots (User Guide)