Workaround for missing migrate-deregister command in Drush 10.3.1? I'm trying to migrate and upgrade a site from Drupal 7 to Drupal 8 (version 8.9.1). The migration cannot be accomplished with the UI, since I need to make some changes to the content types and taxonomies during the process. I followed the advice at https://www.drupal.org/docs/upgrading-drupal/upgrade-using-drush and generated individual migrations using drush migrate-upgrade --configure-only so that I could use the YAML files it generates as the basis for a custom migration module.
After I built my module, I ran drush migrate-status (alias drush ms). I can see that my migrations are registered, but the ones generated earlier by drush migrate-upgrade are also there. In the past, removing them would be a simple matter of using drush migrate-deregister (alias drush mdreg), but that command, strangely, is no longer available in the current version of Drush (10.3.1).
How do I deregister the migrations that were generated by drush migrate-upgrade --configure-only? I am assuming that if I don't deregister them, my content will be migrated twice, and I don't want that to happen.
Those migrations are stored as configuration.
When you run drush migrate-upgrade --configure-only the command created various migrate_plus.migration.* config data related to your D7 migration.
If you remove the config, you'll remove the migrations. For example:

If you have config captured in a sync directory, do a drush cex, remove all migrate_plus.migration.* yml files, then drush cim. Drush will drop all migration configuration on import.
If you'd rather delete the migration directly, drush cdel will let you specifically delete a migration.

Even if you delete the migration config, some migration metadata is still kept around in the database (see migrate_map_ &amp; migrate_messages_ tables). They'll be re-used if you re-import migration config. If you're finally finished with your migrations, you can delete them.