Sorry for the trouble. I created a new test illustrator project and tried it, and confirmed that the .jsx file you attached can export JSON even in v26.0.3. Could you send us the ai file which has the project via email? : contact@esotericsoftware.com
Please briefly mention this forum thread URL in the email so that we know the context. Then we can have a look at what's going wrong.
Regarding the list of scripts, I have also tested this on my PC and confirmed that IllustratorToSpine can be displayed on the list correctly. I believe it is not the issue of the script side, so please ask about it on Adobe’s support community.