SC II no longer offers 2560x1080 (21:9) support on my system. My system was just updated to the Windows 10 Insider track build 19013 and when I launch SC II, it comes up in 720P settings. The largest option under settings for Full Screen is 1920x1080 (1080p). I had previously been running in 2560x1080 with no issues.
Windows 10 build 19013
Nvidia GTX 1050Ti, driver version 441.12
i7 @ 4.4GHz
16 GB RAM