I recently updated to PHP 7.3 on my server. I think both exec and shell_exec are disabled by default, so I changed this in the php.ini after I ran into this error.
When I tried adding a new admin user, by the time a password is being generated, the following error appears in an endless loop: [ERROR] Warning: exec() has been disabled for security reasons
It's not a problem for me personally but I think it's a good idea that this is mentioned in a "system requirements" paragraph in the docs. I know you are still working on the docs btw. ;)