The author of the YouTube channel Modern Vintage Gamer published a video in which he launched the PS2 classic on the Xbox Series S. This, of course, is not about full-fledged backward compatibility, but about emulation.
Microsoft consoles have a developer mode where you can enable apps with the UWP (Universal Windows Platform). One such application is the RetroArch emulator platform, which includes PCSX2, a PS2 emulator.
The author of the channel warned that PCSX2 is under development, so the current version is full of bugs. However, emulation of PS2 on Xbox Series looks promising already. You can safely play Ico, Metal Gear Solid 2 and Silent Hill 2.
Modern Vintage Gamer did not test the emulator on Series X, but assumed that everything would be the same on Microsoft’s older console. PCSX2 primarily loads the processor, and the CPUs of both models are similar.
PS5 doesn’t do that.