Oh, they still do that with their new hardware. The machine comes with x amount of processor cores, but you can't use any of them without paying. How much you pay depends on the "MSUs" you agreed to, MSU being a proprietary measurement system by IBM.

Other software you run is billed relative to your MSU tier. So, if you run z/OS then your cost will be higher if your machine has more MSUs. A weird quirk of this is that there is thing called "IFLs" (Integrated Facility for Linux) which, when I when I first heard of them, I thought was a separate processor designed for for linux. However, it is not. It is actually the same as the regular processors that run z/OS etc, the difference is that is is licensed exclusively for running Linux (or like z/VM to run linux counts too). The reason for this is to enable shops that want to run linux and needed extra horsepower to do so, but didn't want their z/OS bills to go up because they purchased more MSUs. So, despite buying more of the processor capacity within the mainframe, it doesn't count towards the "MSU" number that impacts the cost of various software because you are using with one type of software vs another type of software.