Hi @anon86287507, I don’t have any update on this topic at the moment. Mandatory Custom Fields Filling is slightly different as it relates to filling all custom fields before a task can be added to a project - while in your case, you need all custom fields to be filled before marking the task complete, which is slightly different - so I’ll keep them separate for now 
I’ll make sure to keep you posted as soon as I have some news on this topic, thanks again for sharing your feedback with us!