They didn't upgrade to PCIe 4.0 until the P5800X series. As far as I know, the P5800X series was only offered in U.2 form factor, however. And in a panic, I already bought one of those last fall. I didn't expect we'd be given so much advanced warning.
These were plausible for high-end server usage. But these and the 900P and 905P for PC use were throwing money away. While in some niche areas they had superior performance, they were way too expensive for consumers against their rather limited benefits. No surprise INTEL ended this.