Shift scroll doesn't horizontally scroll with any external mouse I used to use Shift+scroll to scroll horizontally in the past. However, when I tried this on my MacBook running macOS Catalina, it didn't seem to work. I tried this with multiple external mouses—the result is only a horizontal scroll, irrespective of whether the shift key is held down. I tried this in Safari as well as some other apps. 

I did some quick research, and it seems as shift-scrolling should scroll horizontally, as seen here, here, and here. 

Could this possibly have been a removed feature in Catalina? If so, what are my options for getting it back?
The issue was that I was using Mos to enable smooth scrolling on my Mac.

However, it removes the ability to scroll horizontally using the shift key by default. To reenable this behavior, go to Mos Settings > Advanced and then select "Shift" as the horizontal scrolling key. That should do it!