You can check the status of various Apple online services here:
Your iCloud storage is almost fullRecently (2016?) Apple seems to have started sending lots of fear-inducing emails with the subject line:
Your iCloud storage is almost full
Have received these emails recently (in 2016):
iOS 9.x has a UI in the Settings app (iCloud option) for enabling various iCloud Photo features:
iOS 9.x has a UI in the Settings app (Storage option) for enabling various backup features, which use iCloud:
Note: the "Photo Library" backup option is different from the "iCloud Photo Library" and "My Photo Stream" options in the explicit iCloud Photos preferences above.