Uninstall Multiversion and Workspace from Drupal 8. This is an update hook which successfully uninstalled the module from our fairly complicated site.

modulename.install.php

<?php

/**
 * Uninstall Multiversion and related modules.
 */
function modulename_update_8000() {
  
  // !!! IMPORTANT !!!
  // First, make sure to check and see if any of the below entities have a
  // value of NULL in "_deleted" or "workspace" columns, because if so, 
  // you'll want to update that. See this issue for more information:
  // https://www.drupal.org/project/multiversion/issues/3144466
  
  // These are all of the entity types that we CAN run disableEntityTypes() on
  // without any data loss. The others we will handle separately below.
  $entity_types_to_disable = [
    'redirect',
    'menu_link_content',
    'file',
    'shortcut',
    'taxonomy_term',
    'media',
    'crop',
  ];
  $multiversionManager = \Drupal::service('multiversion.manager');
  foreach ($entity_types_to_disable as $entity_type_id) {
    echo "Disabling Multiversion for " . $entity_type_id . "\n";
    $entity_type = \Drupal::entityTypeManager()->getStorage($entity_type_id)->getEntityType();
    $multiversionManager->disableEntityTypes([$entity_type_id => $entity_type]);
  }

  // Delete all the Workspace entities so that the module can be uninstalled.
  $entity_types = [
    'workspace',
    'workspace_pointer',
  ];
  foreach ($entity_types as $type) {
    $defs = \Drupal::entityManager()->getDefinitions();
    if (!empty($defs[$type])) {
      $controller = \Drupal::entityManager()->getStorage($type);
      if ($entities = $controller->loadMultiple()) {
        $controller->delete($entities);
      }
    }
  }

  // Uninstall Multiversion and dependent modules.
  $modules = [
    'workspace',
    'multiversion',
    'replication',
    'relaxed',
  ];
  \Drupal::service('module_installer')->uninstall($modules);

  // Now we have to manually remove the Multiversion-powered field storage
  // definitions for the entity types that we could NOT run disableEntityTypes()
  // on. This way we accomplish the same thing but without the migration that
  // Multiversion does, which was causing lots of corrupt and lost data.
  $entity_definitions = \Drupal::keyValue('entity.definitions.installed');
  $definitions_to_update = ['block_content', 'node', 'paragraph'];
  foreach ($definitions_to_update as $definition) {
    $fields = $entity_definitions->get("$definition.field_storage_definitions");
    unset($fields['workspace']);
    unset($fields['_deleted']);
    unset($fields['_rev']);
    $entity_definitions->set("$definition.field_storage_definitions", $fields);
  }

  // Now we need to delete any entities of those entity types which were
  // marked as _deleted. This is another thing that disableEntityTypes() would
  // have done for us if we could have used it on these entity types.
  $database = \Drupal::database();

  $tables = [
    'node_field_data' => [
      'alias' => 'n',
      'id_field' => 'nid',
      'entity_type' => 'node',
    ],
    'block_content_field_data' => [
      'alias' => 'b',
      'id_field' => 'id',
      'entity_type' => 'block_content',
    ],
    'paragraphs_item_field_data' => [
      'alias' => 'p',
      'id_field' => 'id',
      'entity_type' => 'paragraph',
    ],
  ];

  foreach ($tables as $table_name => $table_info) {
    $query = $database->select($table_name, $table_info['alias']);
    $query->fields($table_info['alias'], [$table_info['id_field']]);
    // We only need to delete the entities where _deleted = 1, meaning they
    // were already manually deleted but Multiversion was keeping them around
    // because that's what Multiversion does.
    $query->condition($table_info['alias'] . '._deleted', 1);
    $result = $query->execute();
    $entity_ids = [];
    foreach ($result as $row) {
      $entity_ids[] = $row->{$table_info['id_field']};
    }
    $storage = \Drupal::entityTypeManager()->getStorage($table_info['entity_type']);
    $entities = $storage->loadMultiple($entity_ids);
    $storage->delete($entities);
  }

  // And finally, delete the Multiversion-powered fields on the entity tables.
  // Not doing this won't actually cause any problems, but they are useless
  // so we may as well get rid of them.
  $schema = \Drupal::database()->schema();
  $tables = [
    'node_field_data',
    'block_content_field_data',
    'paragraphs_item_field_data',
  ];
  $fields = ['workspace', '_deleted', '_rev'];
  foreach ($tables as $table) {
    foreach ($fields as $field) {
      $schema->dropField($table, $field);
    }
  }
  
  // Rebuild the taxonomy_index table in case the uninstallation
  // process cleared it out.
  $query = \Drupal::entityQuery('node');
  $ids = $query->accessCheck(FALSE)->execute();
  $storage_handler = \Drupal::entityTypeManager()->getStorage('node');
  // If you have a lot of entities on your site, you may need to use
  // array_chunk() here to split this up to avoid loading all nodes
  // at the same time.
  $entities = $storage_handler->loadMultiple($ids);
  foreach ($entities as $entity) { 
    taxonomy_build_node_index($entity); 
  }
}

Great work, Mike! I am running into an error when running this against my app running core 8.6.9 plus a very old version of multiversion:

>  [notice] Update started: ukh_mv_uninstall_update_8100
>  [warning] count(): Parameter must be an array or an object that implements Countable ContentEntityStorageTrait.php:265
>  [warning] count(): Parameter must be an array or an object that implements Countable ContentEntityStorageTrait.php:301
>  [error]  Drupal\Core\Entity\EntityStorageException: Table mapping contains invalid field workspace. in Drupal\Core\Entity\Sql\SqlContentEntityStorage->mapToStorageRecord() (line 962 of /Users/MyUsername/Sites/devdesktop/redacted-dev/docroot/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php). 
>  [warning] Attempt to assign property 'new_edit' of non-object ContentEntityStorageTrait.php:179
>  [error]  Drupal\Core\Entity\EntityStorageException: Table mapping contains invalid field workspace. in Drupal\multiversion\Entity\Storage\Sql\ContentEntityStorage->save() (line 180 of /Users/MyUsername/Sites/devdesktop/redacted-dev/docroot/modules/contrib/multiversion/src/Entity/Storage/ContentEntityStorageTrait.php). 
>  [error]  Table mapping contains invalid field workspace. 
>  [error]  Update failed: ukh_mv_uninstall_update_8100 
 [error]  Update aborted by: ukh_mv_uninstall_update_8100 
 [error]  Finished performing updates. 

Any idea what this could be getting hung up on? My db tables with the workspace column are these:
DB Tables

It takes aroun 1.5 hours to throw this error, so I assume the script is actually working but my app is configured slightly differently or the antiquated version of Multiversion is causing the issue.