I would prefer a more permissive license, such as MIT or Apache. Zulip is similar software, produced by a commercial company, and it is MIT licensed. The primary reason for this is that it isn't always clear with copyleft licenses where the boundary is between being a new separate work that uses the GPL software API, or when it is a derivative work of the GPL software. Also, as they are shutting down, there isn't really any reason for them to worry about some one else using the code in a permissive way.

(That being said, a copyleft license is miles ahead of the CC NC license for software!)