Because why should I send full collection object in the request if I only want to remove its image? It doesn't make sense.
Here is link to reference https://shopify.dev/docs/admin-api/rest/reference/products/customcollection#update-2021-04
Did you try the PUT to see if it functions as expected? From what I can tell it functions correctly as described, but if you notice a case where it doesn't, please provide us with an x-request-id response header so we can take a look.
Yes, I tried and it works.
But why I should send a whole object(with all properties) to remove only the image? Why should I send published_at property when it is read only? Did you see the request body in the example?
To remove collection image, It is enough to send id and image properties.
Please correct/update reference. There are many similar errors/bugs in reference.
Do you see the difference and the nonsense of the example?