I have this issue too. I have an 21:9 3440x1440 ultrawide monitor.
Game settings do detect the resolution correctly and the correct aspect ratio is shown in menus. Actual in-game map however does not fill the entire screen and black bars are visible inboth sides.
The issue is not only with ultrawide monitors, but also with any monitor with aspect ratios other than 16:9. If say 16:10 or 4:3 screen is used, it has black bars on top and bottom. The in-game view seems to only support 16:9 aspect ratio.
As this should be a "remastered" game, I'd expect it to function with modern PC hardware. The game was after all advertised with high resolution graphics. Most other games released in recent years also do support ultrawide just fine (specially other RTS remasters like Age of Empires I&II Definitive Editions).
Based on recent hardware surveys not that many people have those 4K screens either (for which the game is advertised). Actually there are practically as many ultrawide users as there are 4K users, at least based on public Steam HW survey. When other non-16:9 resolutions are taken into count, there are acutally more users with incompatible aspect ratio screens than there are users with advertised 4K monitors.
https://store.steampowered.com/hwsurvey/Steam-Hardware-Software-Survey-Welcome-to-Steam
