Contrary to the changelog of version 4.2.0 one cannot change a channel to private/public.
Execute:
./bin/platform channel modify [team]:[channel] --[public|private]
The channel should now be public/private.
Error: Feature requires an enterprise license. Please contact your system administrator about upgrading your enterprise license.
Maybe it was before only possible with the enterprise edition (or (still) is?), but then the documentation is misleading.
Thanks very much for the report. This was a bug and has been fixed for Mattermost Server 4.3, which will be released on 16th October, where this CLI command will be available in all editions.
Most helpful comment
Thanks very much for the report. This was a bug and has been fixed for Mattermost Server 4.3, which will be released on 16th October, where this CLI command will be available in all editions.