What is happening: In preparation for work upstream on the production wiki databases, the Wikireplica service needs to drop some columns from the views used by Toolforge and CloudVPS users.
The columns being dropped are:

* archive.ar_text_id
* archive.ar_content_model
* archive.ar_content_format

* revision.rev_text_id
* revision.rev_content_model
* revision.rev_content_format

NOTE: revision.rev_content_format and revision.rev_text_id are only relevant when loading serialized blobs from external storage, which is
not possible from the Wiki Replicas. These columns are removed without replacement.

These columns currently contain stale data in both the replicas and the production databases. The actual data used in production was moved
entirely to the "slot" and "content" tables on 2019-11-18 (<https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/551551/>).

Information about migrating tools to the new schema is available in the description of this task: https://phabricator.wikimedia.org/T174047

Additional information about the overall project and changes can be found here:
https://www.mediawiki.org/wiki/Multi-Content_Revisions/Database_Schema

The columns will continue to exist in the the revision_compat and archive_compat tables as a stop-gap to keep a tool that relies on those fields from completely breaking while you work on updating to the new schema. These two views are expected to perform poorly because of they include joins against the content and slots tables. Please only use those if you need them and will take longer to finish refactoring your code to the new schema.
When is this happening: This will take time to run across the replicas and databases, possibly over the course of a few days, beginning 2020-05-25. There is a process of depooling of the servers that will take place to allow the changes to take place.

What should I do: Between now and the 25th of May, stop using the fields we are removing. If you don't already, make sure you use the slots and content tables instead.

Progress on this action will be tracked on this Phabricator task - https://phabricator.wikimedia.org/T252219.  