I have tried this method and these rules to solve my problem, but it has no effect. I tried making the json in /etc/Chromium/policies/managed and also /etc/opt/chrome/policies/managed. I think it fails because I am using the snap version of Chromium and maybe the config files are read from somewhere else. So how to disable the Guest mode option in the snap version of Chromium? Maybe there is an easier way?
Running 20.10
Edit: To clarify, this method works for Chromium based browsers installed without snap.
For the moment, this is not as such possible (i.e., without having to create and install your own custom snap of chromium). See this bug report. A fix is committed, so you should be able to implement that soon.