After updating to 15.19, seems that Garmin have removed the ability to customize data screens (which is annoying in its own right, but the lack of Navigation fields made customization irrelevant). However, the same happened for the Map screen (not the one that I can add as a data screen that shows up always, but the one that I can toggle via the Navigation settings). Seems that there's only the option to turn "data field" on/off, but the data field itself is not editable. Currently for me it's set to heart rate, which is something that I set a while ago, but I don't what that one to show up.