The Add new button adds a button to the drop-down for each enabled feature.
So disabling models, removes the 'Add new Model' option from the drop-down.
Strains on the other hand does not have a configurable setting, which makes it always added to the drop-down.

To solve this, a new setting should be added which adopts the same value as the organisms_enabled config.
So when organisms in enabled, 'Add new Strain' will show and when organisms is disabled, 'Add new Strain' is not add to the drop-down.