Filipe Espósito, for 9to5Mac:

[…] we have found evidence that Apple is working to completely replace the Messages app on macOS with a Catalyst version, based on iOS 14 code.

I assume this will mean the end of AppleScript support for Messages. I don’t use it, so I won’t miss it, but a lot of people will be sad to see it gone. I am however hoping that it will still be possible to navigate Messages by using just the keyboard, which I do right now — ⌃⭾ moves me down the list and ⌃⇧⭾ reverses direction. Since the move Catalyst is to get feature parity between platforms, I hope they add keyboard shortcuts for stickers, tapbacks, and other features. Tapbacks are currently possible but require using the trackpad or mouse:

Send a Tapback: Click and hold a message, then choose a Tapback. If your Mac has a Touch Bar, you can also click a message, then tap a Tapback in the Touch Bar.