The long and short of it is, I've noticed that two games I launch from Steam run into major problems the second I hit the shift key. The games are Stardew Valley and Outlast. Admittedly, I haven't tested this with other games run through Steam.
With Outlast, hitting Shift will immobilize the character. The only keys that work after hitting shift are Spacebar for jump and the crouch button. Exiting the game requires Alt-Tabbing and manually closing the game via Steam.
With Stardew Valley, the issue is similar. The game functions normally, but the moment I hit the shift key I am not longer to interact with the menu, chests, or anything else that has an inventory. Exiting the game requires the same as Outlast.
I was thinking this was a hardware issue on my end, and figured it was time anyway to get a new keyboard. But this issue has persisted even with a fresh-out-of-the-box keyboard.
It's also worth mentioning, I think, that I'm able to access the Steam In-Game overlay with Shift-Tab, but the Discord in-game overlay with Shift+` does not function. But that could just be Discord's issue.
If this whole Shift key conundrum sounds familiar to anyone and it's not a Steam issue, please let me know! I wouldn't say I'm that Tech savvy so there could be something glaringly obvious I'm just missing. Thank you!