Not sure how long this has been the case (likely since the very beginning of share sheets), but this feature is definitely new to me, thought it worth sharing.
- Bring up an iOS app, then bring up a share sheet. In the Safari app, bring up a web page, then tap the share icon (square with up arrow) to bring up the Safari share sheet.
- Press and hold an icon until it grows slightly, then slide to the left or right to move it to a new location.
This technique works with both the app shelf and the tool shelf.
To see this in action, watch the video in the tweet below: