ArduinoJson 5.13.4 has just been released. It fixes a long-standing bug with the Particle Web IDE.
Because I didn’t properly publish the previous version of ArduinoJson to the Particle ecosystem, it was possible to use the library with the CLI (the command line interface), but not with the online IDE.
If you tried to load ArduinoJson, it showed “Loading…” and hung there.
Fortunately, this problem is fixed, and you can now include ArduinoJson in any Particle project.