When creating a custom field option, it would be nice to be able to tick off a check box (or something) to add the custom field to all forums, rather than highlighting all of them manually. Just a friendly suggestion.
While not the same, on a desktop, you can generally click the multi-select and then press ctrl+A (or similar) to select all of the entries, though I don't think there's really an expectation that you'd add a custom field to every forum.
Yes I'm aware of Ctrl A. Actually there are instances for all forums to have some custom field. Another platform has this ability so it may be something necessary to more forums than we think. IDK.