For those following along from the sidelines, Drupal 8 core has 3 modules that make up the migrate sub-system. For the past few years, the community has been working very hard to get these to a stable state an out of their "experimental" designation. Drum role please... as very soon the first of these modules is going to do just that. Go stable. The migrate module (also known as the API module) should have its last critical release blocker committed in the next few days. At which point, this module can be called stable.
What does that mean? Personally, it means a lot of my efforts have paid off. More broadly for the community, folks in contrib and others building their own custom migrate solutions can depend that the API is stable. This is a bit of a misnomer, as the API hasn't made any API changes (except for this API addition) in a year or two. But soon it will be official that no API changes are allowed.
What does that mean for migrate_drupal and migrate_drupal_ui? These two make up the rest of the migrate sub-system in core. It means that the community can focus on getting those two modules fully stable without distractions. Those two modules are super close. In similar manner to the migrate API module, there are just a 5 or 6 blockers for calling those two modules stable. The two areas that need attention are in edge cases for multi-lingual (i18n) and the feature to rollback a migration through the UI. Yes, that is all that is left. If you don't have any i18n on your site (and only 20% of Drupal sites do according to d.o. stats) or if you aren't using the migrate_drupal_ui, then you should start planning NOW to upgrade to Drupal 8.
Now is the time to consider your migration to this awesome version of a pretty awesome CMS. Because the entire sub system should be stable in Drupal 8.5. And because if you don't have some very specific edge cases, it will be stable even sooner than that.
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