Looks like Big Sur (I was suspecting Catalyst but it works fine in the Podcasts.app) is breaking default macOS shortcuts. In the official list, text selection shortcuts like the following are listed:
- Shift–Left Arrow: Extend text selection one character to the left.
- Shift–Right Arrow: Extend text selection one character to the right.
If you try to use them in the new Messages.app they don’t do anything. It’s working perfectly fine in the old Messages.app on Catalina.
(Feedback submitted: FB8895454)