Control Z actually just is undo now?

It's not that simple. Photoshop is almost 30 years old. When Photoshop was originally released in 1990, the shortcuts were consistent with what other applications were doing. Most Mac applications only had one undo, so some applications used Command-Z as an Undo/Redo toggle.

But the standards changed over the years. In 2000 Apple first released OS X, and OS X took over the Command-H and Command-M shortcuts for Hide (application) and Minimize (window). But Photoshop had been using those shortcuts for 10 years. Mac users started complaining that Photoshop shortcuts were not consistent with OS X, but if Adobe changed the shortcuts, Photoshop users would complain that shortcuts were getting changed. So should the shortcuts follow Photoshop tradition, or new Mac standards? For Command-H, Adobe now lets the user decide at startup and in a preference.

A similar thing happened with Command-1. It used to be a channel shortcut, but over the years more applications started using Command-1 to mean 100% zoom. So Photoshop also followed this standard, but there were many complaints about the channel shortcuts getting changed. Again, which is more important: Photoshop tradition, or cross-application consistency? As with the other shortcut, Adobe lets you decide in the shortcut preferences.

The earlier Photoshop made both of those changes, the better, so more users would be trained on the updated shortcuts than the outdated ones.

Same with the new Undo shortcuts. Photoshop used to be perfectly in line with the single undo model, things changed over the last 29 years as multiple undo evolved, now Photoshop has adapted and is once again consistent with others. If you don't like the change, that's OK, Photoshop give you the choice to switch it back, as JJMack posted.