Don't delete it. We have many legacy projects that still use this functionality. I propose to do this deprecate.

@AntonLGVS Restored the method I deleted, thanks.
If it's still needed to support legacy projects, how about making this method use transferTo() in InputStream?
like below

public static int copy(InputStream in, OutputStream out) throws IOException {
    Assert.notNull(in, "No InputStream specified");
    Assert.notNull(out, "No OutputStream specified");

    int byteCount = (int) in.transferTo(out);
    out.flush();
    return byteCount;
}

I'm not sure Spring should keep StreamUtils#copy method. For Utils such as copy method, It looks like more proper to use apache commons or guava libraries for legacy projects.

@yaboong Thanks. It's a good idea to use transferTo() within the copy method.

@bananayong Why use an external library when Spring provides a basic set of methods. If you want to delete, you must first make a deprecate to give time for the transition. Otherwise, updating the spring package will break everything.