Is EasyKAD™ Kanban Boards available for Jira Server?

No.  EasyKAD™ is only available for Jira Cloud licenses. It is available for both Jira Core and Jira Software.

Atlassian no longer sells new licenses for Jira Server.  Existing Jira Server customers can receive maintenance and support for up to three years until February 2, 2024.   Jira server customers can purchase EasyKAD™ app when they move to Jira Cloud.

For more details about Atlassian’s deprecation of Jira Server visit:
https://www.atlassian.com/migration/faqs