You're conflating two things. Is it that they don't support the features or is it that they're proprietary?

It’s a case of nvidia refusing to support Linux properly, rather than the other way around - nvidia have been attempting to force their own APIs on Linux graphics developers rather than fit in with what everyone else (Intel, AMD, and all ARM GPU developers) have been doing.

The result is that anyone doing low-level graphics on Linux would have to support two entirely separate, very complex APIs - one for everyone but nvidia, and then one for the proprietary nvidia drivers. Unsurprisingly, not many people are up for that.

Realistically, nvidia just don’t care about Linux outside of very specific use cases where they’re working with the system integrators directly, the same as Broadcom and a few others. If you’re building a Linux system without their support, you’d be well advised to stick with GPU developers that actually work with Linux developers.

I'm not sure an open source Nvidia driver is necessary for this. The problem mostly seems to be that Nvidia does not support standard interfaces all other open source and closed source drivers support. Nvidia was very late to the party to support kernel modesetting, which is supported by all other modern graphics drivers.

Nvidia refuses to support GBM, a standard way of allocating memory for graphics. Either they refuse this out of spite, or (more likely) because it fundamentally doesn't fit the architecture of their closed source driver. They are so desperate to push their alternative, EGLStreams, that they have lobbied Gnome and even created patches for KDE to be able to run Wayland on these DEs. This is not some closed source vs open source thing, since AFAICT the closed source Mali driver seems to support GBM and Wayland implementations that need it.

Therefore, the problem isn't that Sway doesn't support Nvidia. The problem is that that Nvidia doesn't support the standard ways to do graphics on Linux. This something Nvidia can in principle remediate themselves if they want it enough.

Probably, but given the relative levels of adoption of Nvidia graphics cards and Sway, I suspect that he's doing more to limit adoption of Sway than he is of encouraging Nvidia to change their behavior.

Seeing as even Linus was unable to "persuade" them, this means "someone will write a fork with nvidia driver support eventually"

No, the problem is that the driver uses it's own API called EGLStreams instead of GBM, which everyone else (including Sway) uses. Open-source is not a requirement for Sway support, just GBM support.