WSL1 is just an upgraded Interix, which had been with Windows since at least 2000, maybe even NT4. All the APIs are there, porting WSL1 on 7 should be not a problem at all.
WSL2 is just a virtual machine. Hyper-V was first shipped in Windows 8, which is not too far from Windows. But things are even more bright. Qemu already had the "kqemu", which what eventually became KVM when merged into the Linux kernel, and it worked _even on windows NT 4_, and if course it worked on 2000 and XP.
So porting Hyper-V/WSL2 as a driver onto Windows 7 should be even less of a problem than WSL1.
WSL2 is just a virtual machine. Hyper-V was first shipped in Windows 8, which is not too far from Windows. But things are even more bright. Qemu already had the "kqemu", which what eventually became KVM when merged into the Linux kernel, and it worked _even on windows NT 4_, and if course it worked on 2000 and XP.
So porting Hyper-V/WSL2 as a driver onto Windows 7 should be even less of a problem than WSL1.