Ok, two more changes I've noticed, and one that is definitely new with 10.4
First, this is new with 10.4. Starting with 10.0, if you used list view for your apps, pressing the crown to show the list of apps always brought you to the top of the list, rather than to the last place you left the list, as with all versions that supported list view prior to 10.0. 10.4 now remembers where you were in the list when you press the crown.
New to 10.4 (I think) is the behavior of a double-press of the crown. Prior to 10.0, double-pressing the crown brought you automatically to the last app you opened, and doing it again brought you to the app you just left. It was an easy way to switch between two apps.
10.0 changed this, A double-press of the crown brought you to the multitasking screen, showing thumbnail cards of the apps that you've opened in reverse chronological order, similar to swiping up halfway on a FaceID iPhone or double-clicking the home button on a home button iOS/iPadOS device. From here you could swipe left and tap a control to force-quit an app, or tap an app to go right back. Starting with 10.4, double-clicking with 10.4 brings you to the list, but after about a second it brings you directly to the last opened app. So double-clicking the crown is back to switching between two apps, with a slight delay on the multi-tasking screen.