Brian, it's true that not all of the sources would be familiar with all of the symphonies of the composers you listed, because Schnittke and Henze wrote their last symphonies in 1998 & 2000 respectively, so their last symphonies are newer than some of my sources. But it's not necessarily the case that if they didn't list these works, then they must not have heard them. What you think is good won't always be the same as what another person thinks is good, and I think most of the sources are basing their choices on more than just how good they think the work is. What I'm trying to show on this list is the general consensus for what the greatest symphonies are.
Many of the symphonies by the composers you listed are fairly recent, and sometimes it takes time for a work to be recognized as great, even if it's a work that is well enough known. That's true of Mahler's symphonies. They weren't generally recognized as great right away, not because they were unknown, but because at first, most people didn't understand them.
Thanks for the response, Brian. There are certainly people who would have critiqued Mahler's symphonies long ago, so why was their 'greatness' not recognized by those (I assume) musicologists/music critics at that time? I don't think anyone can say, so I don't think it really matters in the least. The bottom line is that it all comes down to interpretation according to perception, and aesthetics. I just hope people realize that lists are nothing more than a starting point for the exploration of what many of us think is amazing music. Thanks for the excellent work on the lists, Brian. I'm sure anyone who's been here recognizes and appreciates your outstanding effort.