Pretty much every major release fully supports 21:9 natively. I posted(just a few posts above yours) only three major examples in 2016 off the top of my head that didn't quite support it.
-Dragon's Dogma (fixed within hours after launch thanks to a community modder with a simple copy/paste file, so full 21:9 if you do the 30s of work)
-Skyrim SE (Can play just fine in 21:9 by editing an .ini file, but the UI doesn't scale correctly so you miss some info that is cut off in inventory, hoping for patch support from Bethesda or the community to apply a proper fix...)
-Overwatch (Blizzard took it out for competitive reasons...not sure if they reinstated?)
It's actually more difficult to think of games(at least those deserving of GAF attention) that
don't support it. Hell, even GAF darlings
Trails in the Sky and
Stardew Valley support 21:9 natively on Steam.
Speaking of Steam, there is a
community curator group that keeps abreast of 21:9 supported games that come to Steam.
Seriously, this thread should have begun and ended with 21:9 monitor recommendations. Just be mindful the monitors aren't cheap, but next to a videocard upgrade, 21:9 is the best investment a PC gamer can make for a rig if you appreciate being drawn into as much lush visuals as your peripheral vision can handle currently.