Hi @C27618681pmp5, thank you for reporting this issue.
@Dan Ebberts is correct—ExtendScript has never natively supported the JSON object or its asscociated methods. When you have seen the JSON object available, it is because the Libraries panel has polyfilled it in the global scripting namespace. Other third-party-scripts maybe have also done the same. If you open the Libraries panel, I expect you'll find that JSON is again available in scripting.
That said, it is definitely best practice to have your script load the JSON API itself in case the Libaries panel hasn't been opened in the current session of After Effects.
Cheers,
- John, After Effects Engineering Team