Drupal 8.0.0 replaced the earlier, in-place upgrade procedure for major version upgrades with a migration-based solution for core and contributed modules. Several modules serve this need in core: The Migrate module provides a general API for migrations, the Migrate Drupal module provides API support for Drupal-to-Drupal migrations, and the Migrate Drupal UI module offers a simple user interface to run migrations from older Drupal versions.
A lot of work has gone into making migrations more complete since the initial 8.0.0 release, including for multilingual sites with various configurations. Drupal-to-Drupal migrations are still not wholly complete (especially for Drupal 7 sources). However, lots of real-life use has validated the choices we made with the base Migrate API, and key architectural improvements have been completed already. An increasing number of contributed modules rely on it for their migrations.
Based on this stability and success, the Migrate subsystem maintainers and Drupal release managers have agreed the Migrate API (the Migrate module) now has beta stability! The change took effect in Drupal 8.2.x with 8.2.5 and will apply to 8.3.0 onwards as well.
What does this mean for sites and developers relying on the Migrate API?
Beta experimental modules are considered API- and feature-complete and beta modules are subject to the beta allowed changes policy. This means that module and migration developers can rely on the API remaining stable from now on! This also means that the focus with Migrate API is on fixing critical issues as well as bug fixes and contributed project blockers, if they are non-disruptive, or if the impact outweighs the disruption.
Note that Migrate Drupal and Migrate Drupal UI are still alpha stability, so API changes may still happen in these modules. Completing the Drupal-to-Drupal migration path and getting Migrate Drupal to beta stability is the next priority, so your help with missing and incomplete migrations is welcome!
If you want to get involved, the migration team is meeting every week at alternating times. The team has Google Hangouts at Wed 1pm UTC or Thu 1am UTC on an alternating weekly schedule. The #drupal-migrate IRC channel is also used as a backchannel.