Migration from Subscriptions module on D7

Nice to see that usecase. Not sure if I can be of much help here since I'm not sure about the former subscription data structure. I could certainly provide some feedback on how the subscription data is stored by DANSE, though. @DamienMcKenna let me know if and what you would need from our side, please.

WIP source plugin.

<?php

namespace Drupal\my_migrate_stuff\Plugin\migrate\source;

use Drupal\migrate\Annotation\MigrateSource;
use Drupal\migrate\Plugin\migrate\source\SqlBase;
use Drupal\migrate\Plugin\MigrationInterface;

/**
 * Migrate source for the Subscriptions data.
 *
 * @MigrateSource(
 *   id = "d7_subscriptions",
 *   source_module = "subscriptions"
 * )
 */
class UserSubscriptions extends SqlBase {

  /**
   * {@inheritdoc}
   */
  public function query() {
    $query = $this->select('subscriptions', 's');
    $query->join('subscriptions_user', 'su', 's.recipient_uid = su.uid');
    $query->fields('s', [
      'sid', 'module', 'field', 'value', 'recipient_uid',
      'send_interval', 'author_uid', 'send_updates', 'send_comments',
    ]);
    $query->fields('su', [
      'uid', 'digest', 'secure_links', 'send_interval',
      // Ignore these two columns as they're duplicates for our use case.
      // 'send_updates', 'send_comments',
      'send_interval_visible', 'send_updates_visible',
      'send_comments_visible', 'autosub_on_post', 'autosub_on_update',
      'autosub_on_comment', 'send_self', 'suspended',
    ]);
    $query->orderBy('s.sid');
    print_r((string), $query);

    return $query;
  }

  /**
   * {@inheritdoc}
   */
  public function fields() {
    $fields = [
      'sid' => 'sid',
    ];
    return $fields;
  }

  /**
   * {@inheritdoc}
   */
  public function getIds() {
    return [
      'sid' => [
        'type' => 'integer',
        'alias' => 's',
      ],
    ];
  }

}

Migration config for the "update" data:

id: mysite_users_subscriptions_update
migration_group: mysite
label: 'Users: Subscriptions: Update'
audit: true
migration_tags:
  - LBL
  - Users
source:
  plugin: d7_lbl_subscriptions
  constants:
    type: 'content'
    subtype: 'taxonomy_term'
    operation: 2
    publish: 'publish'
    update: 'update'
process:
  # None of these values are used.
  # sid: sid
  # field: field
  # recipient_uid: recipient_uid
  # send_interval: send_interval
  # author_uid: author_uid
  # send_updates: send_updates
  # send_comments: send_comments
  # digest: digest
  # secure_links: secure_links
  # send_interval: send_interval
  # send_interval_visible: send_interval_visible
  # send_updates_visible: send_updates_visible
  # send_comments_visible: send_comments_visible
  # autosub_on_post: autosub_on_post
  # autosub_on_update: autosub_on_update
  # autosub_on_comment: autosub_on_comment
  # send_self: send_self
  # suspended: suspended

  uid: uid
  module:
    plugin: default_value
    default_value: 'danse'
  key:
    plugin: concat
    delimiter: '-'
    source:
      - 'constants/type'
      - 'constants/subtype'
      - value
      - 'constants/operation'
      - 'constants/update'
  settings:
    plugin: default_value
    default_value: 1

destination:
  plugin: user_data
migration_dependencies:
  required: {  }
  optional: {  }

Migration config for the "publish" data:

id: mysite_users_subscriptions_publish
migration_group: mysite
label: 'Users: Subscriptions: Publish'
audit: true
migration_tags:
  - mysite
  - Users
source:
  plugin: d7_lbl_subscriptions
  constants:
    type: 'content'
    subtype: 'taxonomy_term'
    operation: 2
    publish: 'publish'
    update: 'update'
process:
  # None of these values are used.
  # sid: sid
  # field: field
  # recipient_uid: recipient_uid
  # send_interval: send_interval
  # author_uid: author_uid
  # send_updates: send_updates
  # send_comments: send_comments
  # digest: digest
  # secure_links: secure_links
  # send_interval: send_interval
  # send_interval_visible: send_interval_visible
  # send_updates_visible: send_updates_visible
  # send_comments_visible: send_comments_visible
  # autosub_on_post: autosub_on_post
  # autosub_on_update: autosub_on_update
  # autosub_on_comment: autosub_on_comment
  # send_self: send_self
  # suspended: suspended

  uid: uid
  module:
    plugin: default_value
    default_value: 'danse'
  key:
    plugin: concat
    delimiter: '-'
    source:
      - 'constants/type'
      - 'constants/subtype'
      - value
      - 'constants/operation'
      - 'constants/publish'
  settings:
    plugin: default_value
    default_value: 1

destination:
  plugin: user_data
migration_dependencies:
  required: {  }
  optional: {  }

Is this still a work in progress or is it ready to use (more or less)?