I believe it was added in one of the earliest betas, but yes it's definitely new to Mojave. Doesn't happen on High Sierra. I think it's a nice change and more in line with how the items wiggle in Launchpad or on the Touch Bar when you customize them. Glad Apple keeps adding/tweaking these little interface details.I have just noticed, when you customize the toolbar (Secondary click and customize) of Apples own apps (Like Mail, Safari and Finder), the icons in the toolbar jiggle.
I don't know when this was added, I just noticed today.