No Backward: The operating system will continue updating itself for supported XYZ hardware. Once any hardware becomes obsolete, it will no longer be compatible, no legacy support.
No Sideward: There will be no compatibility for Linux, Windows, or BSD software at the system level. Compatibility may exist in App-per-VM. By default, only Linux apps are supported in the VM (This case if the system using different kernel like seL4 or so, but if its using Linux then no other OS should be given/cared for compatibility, only the pure/original kernel with the main OS components)
Additionally, if any app running in a VM does not work with a specific Linux version or hardware architecture, it is not the operating system’s responsibility to ensure compatibility (upstream issue).