Why not support 48bpp png images?

Line 512 in 7d12c7d

    
           /* 24 and 32bpp are supported, 48, 64bpp aren't, see http://bugzilla.ximian.com/show_bug.cgi?id=80693 */

Image format is unknown.
We got this error while processing png images.

I checked pngcodec.c and found this comment.

/* 24 and 32bpp are supported, 48, 64bpp aren't, see http://bugzilla.ximian.com/show_bug.cgi?id=80693 */

This URL can no longer be opened, we don’t know why it doesn’t support 48bpp.
Could someone help us answer this question?

Thanks.