Hardware vendors lost the plot in the Winmodem era.
The actual problem with winmodems was them breaking the established software/hardware boundary, and the Linux community not having the resources to follow suit.
Nothing stops someone from taking the free Windows Vivado and making it run on Linux, or taking a Winmodem driver and making it run on Linux, or writing a from-scratch software implementation of a 56k modem that can run on any sound card plugged into a phone line (which is what a Winmodem is), or reverse engineering the bitstream format for these FPGAs and writing a compiler from scratch (or even just the device-dependent backend - the frontend and middle-end can be developed in a more normal way and can be shared with other toolchains). But nobody actually stepped up and did it, which I think is proof that the free software community is a lot weaker than it thinks it is.
You could even do it right now, if you wanted to. You're not, and I'm sure there are good reasons for that. Extrapolate it across all developers, and it's unfortunate that it seems none of them have enough reason to do it. On the flip side, if anyone reading this does suddenly decide they have enough reason to do it... (Incentive: FPGAs are fun to play with!)
The EULA and the fact that the linux versior runs faster & has fewer bugs.
> just the device-dependent backend would be a major improvement and the frontend and optimizer could be shared with other toolchains
That's yosys and it's used by smaller commercial vendors.
> or reverse engineering then bitstream format for these FPGAs
Getting the timing is the hard part (+ good routing afterwards). The bitstream format has AFAIK mostly been reversed. 7 series has mediocre support , but US, US+ and Versal doesn't (probably because they're too expensive for personal usage).