Migrate to Jira Cloud

According to this Atlassian's article, custom fields created by add-ons are not migrated by default.

However, the migration is still possible by the following steps:

Migrate the custom fields' data

After migrate all data from Jira Data Center or Jira Server to Jira Cloud using Jira Cloud Migration Assistant, follow the instructions in this Atlassian's article to migrate Markdown fields to the Jira Cloud instance. Since Jira Cloud does not allow add-ons to add new custom field types, the Jira Cloud instance's custom fields to receive Markdown content must be of the type "Text Field (multi-line)" (screenshots).

Install the Jira Cloud app

Please see the instructions here.

Configure the Jira Cloud app to render the new custom fields as Markdown

Please see the instructions here.