I've heard rumors that Apple is working on a server CPU.
Yes, they are working on it, but since last few years there has been complete silence.
Kind of OT discussion:
Speaking of Apple's role play in the server space, instead of making server solutions themselves, I think it is far more realistic that Apple cooperate with Amazon, Ampere, Google and others to establish ARM as a strong alternative to x86. They all have an interest in this.
With ARM all the big players can build their own custom solutions tailored to their needs in a way they never could with x86.
But there are two parts to this puzzle. You need the server hardware but you also need popular desktop and laptop computers running ARM. If these are not prevalent, then developers will not develop sufficient experience with ARM. Linus Torvalds has been quite clear that your home computer needs to run the same hardware as your server.
But can Apple really capture the server "market space" on their own ?
Actually, technically speaking., Apple servers already existed. Apart from iCloud hosting and Apple's own data centers, Apple offered MacOS servers since at least 1996, when it sold
complete server racks. But yet today, these are little more than
repurposed Mac workstations.
Apple has an advantage here, in that they control the whole widget, but that only applies if BOTH their hardware and software is used. So if we want to get the full advantage of heterogeneous computing power from Apple, we actually need to run macOS in the cloud, not Linux, not FreeBSD and certainly not Windows.
If not, Apple would have to wait for industry standards supported by Linux, BSD, Windows and others to emerge and then tailor their hardware to those standards. This is unlikely to be something Apple agrees with.
Also I am skeptical that Apple would want to sell solutions not running
their software.
This puts potential users in a bind. It helps that macOS is a Unix operating system. That means a lot of Linux and BSD software will run fine on it with minimal change. Yet macOS is not really optimized for server use.
Linux kernel developers are very focused on this and that drives their development efforts. macOS e.g. is highly tuned towards things like low latency to deal with things such as real time audio and video. These are use cases which matter to professionals working on video and audio. That is a deep part of the Apple DNA and heritage.
Sure we can run Linux on macOS through virtualization, but then you have also lost access to Apple specific frameworks such as Core Audio, Core ML etc which utilize custom Apple co-processors.