It's probably unwise to source your memory from Apple.
Intel says that the chips support up to 128 GB. I'm not sure where Everymac gets its data. But if you own lots of macs, it's probably easy enough to check.
Not too new of a thing. A long time history now of Macs that can support X amount of RAM and then reality is more. I often ran above what was possible without issues on all of my Macs. Now though, I don't need more than 32GB of RAM so running 64 or 128 is just not necessary but I don't thin it'll be a problem.
Just because a Mac will take it in doesn't mean it will run stable. We need users and experts to confirm it works flawlessly. I expect in the coming weeks OWC and others will put this to the test to see how stable it actually is. Personally I can't imagine needing 128GB of ram right now but its cool if it does work. 32GB is already a lot for many applications and 64GB is really darn good.
Yeah I ran an old 2008 MBP with 50% more RAM than Apple said was possible with no issues (aside from Nvidia graphics card issues which prompted Apple to switch to AMD and we're still suffering to this day). My grandpa was actually using that machine until recently when I gave him my 2012 retina MBP. So I wouldn't doubt this machine could take more but I doubt I'll personally need more than 64GB before I get a new one.