An external contact provider might lack abilities like flagging or handling attachments. Therefore, those features must be disabled in the UI as well.

New backend configuration:

ui/global/disableFeature/contacts-attachments = true
ui/global/disableFeature/contacts-flags = true
ui/global/disableFeature/contacts-new-folder = true
ui/global/disableFeature/contacts-copy-move = true

Should be combined with:

ui/global/disableUpsell/contacts-attachments = true
ui/global/disableUpsell/contacts-flags = true
ui/global/disableUpsell/contacts-new-folder = true
ui/global/disableUpsell/contacts-copy-move = true