My apologies, I was mistaken. I had in mind apps like iMovie since, like Game Center, it's an app that Apple installs by default.

On 10.11, there are many apps that are not deleteable, but all of them seem to be under a gig (unless I'm missing something because I had, long ago, deleted an app and the OS update didn't restore it).

These include: Game Center, iBooks, Safari, iTunes, Photos, Contacts, Maps, Automator, Font Book, Dictionary, Notes, FaceTime, Chess, Reminders, Photo Booth, and perhaps a few other small-ish apps. These are less than 1 gig. I don't know why Apple did not choose to simply make whatever is so vital in the app bundle a system framework instead.

It's not a big deal to me if these small apps are non-deleteable. I assumed—incorrectly—that the big ones (iMovie, Garage Band, etc.) were non-deleteable as well.