The subscriptions_option plugin works fine for me. Lets confirm you setup.
You have added subscriptions_option to your `$config['plugins']` array. You have set `$config['use_subscriptions'] = false;` to disable the use of subscriptions and then you added `use_subscriptions` to your `$config['dont_override']` array to prevent the user from changing this setting?
If you are currently logged in your will need to logout and back in again once you have set `$config['use_subscriptions'] = false;`
I'll take a look at our IMAP server and see if there is a special-use IMAP extension. Would you mind expanding on how this might help Roundcube "check the box" automatically for folders which are not currently subscribed?
I don't know how much difference it will make to existing users but think of it like this. When a client connects to the server the server tells it "use x folder for deleted, y folder for junk, etc" the client will show those folder and use them as instructed rather than creating its own. The user can override it and configure the client to use the folders they want but its provides a default which would be the same across clients to save the user doing it manually.