Can old iPhones be used as a camera only?

Yes, you can use an older iPhone for photos, and many other single purpose tasks, but it won't give you the same results as a new(ish) iPhone with up to date specifications, but it that isn't crucial you ought to be happy.

You have to define what you mean by old too. Some would consider a two-year old iPhone out of a service provider contract to be old, while the hardware usually gives many more years of service. However, because Apple pushes both the software and the hardware the 'older' iPhones do fall behind. Photos is one area especially because of the constant upgrading of the quality of the cameras and the way iOS works with photos.

Then there is the consideration of what you do with your photographic treasures. You could sync them to a computer in the time honoured fashion, and deal with them as you choose from there, or include the old phone as part of your iCloud ecology and sync the photos to iCloud Photos.

I sense an unasked question also - the iPhone does not need a SIM card to work for you; it just won't receive phone calls in its own right, although it is capable of receiving calls forwarded via Wi-Fi from your current phone.

It could also be used as a webcam with some third-party software from the app store. You didn't specify what sort of camera you were asking about in your question, but it's worth mentioning this potential use. Again, as it has older specifications it won't give the same result as a current iPhone.