As someone who ran Linux on Windows 3.1 era hardware, perfectly well.

Very well, and striped down kernels run on hardware with even less resources today (embedded systems) than 3.1 minimum requirements. If a platform has lost support, and there's sufficient money in it to justify it, you can always go back in time to an older kernel. What really sucks is when a platform vendor maintains their own tree and it gets lost to time.

The real pain is populating the userspace if you need more than just busybox.

There's distros for this: https://github.com/marmolak/gray486linux – it's based on Linux 6.7.

Never used it; don't know how well it works in practice. But it's the type of thing that can be made to work. Looks like some minimum requirements are a tad higher than the minimum of Windows 3.1 though, so it depends a bit on what you mean exactly with "Windows 3.1 era hardware".

Pretty good I would imagine. The first public release of Linux predates Windows 3.1.