I made almost all the plugins, and we've explained extensively before why the server will never update past 1.12.2.
To everyone: Please, stop asking.
1.13+ server performance compared to 1.8/1.12 is terrible, and not sustainable at all for large servers. Almost all of our plugins use NMS, something version dependent that is not easy to update version to version. On top of that, we use custom server versions we developed ourselves (Not Plugins), which are extremely hard to update to other versions, as they require full rewrites of the code in the server's native version.
The server will never update. If you don't like 1.8/1.12, I suggest you play somewhere else.