Hey I get your frustration. I don't know why they don't have it fixed. Some claim it never worked but others know there was at least one beta driver it did work on. This is certainly not the only issue falling through the cracks for far too long.
I gotta wonder 2 things though.
a: Why a monitor maker makes a 21:9 Freesync monitor with known support issues.
b: Why you bought it knowing support issues. If 21:9 was still an issue for me after all this time I would have just gone with a solution that is known to work.
Still definitely doesn't excuse AMD not getting this working. When it's obvious from all the test done over at guru3d it is absolutely hardware supported but the drives don't they even had a registry hack to get it working but the latest Windows 10 1709 killed that. That may even be why AMD doesn't focus on a fix for this. Maybe too many variable changing on the MS side of things. But it wouldn't take much for AMD to answer what is the current state of things. I don't see where they have done that in quite a while.