SiFive stopped making and selling the Unmatched (FU540) board 2 years ago. The only Linux capable board that SiFive currently sells is the Unmatched (FU740) board.
With both the fu540 and fu740 processors, all of the firmware is open source. There are a few instructions in a mask rom executed at power on reset, but they just read the value of the boot select switches and branch to the selected address. After that, the executed code is all user controllable, and is all upstream in u-boot, opensbi, and the linux kernel.
The Unmatched board has a pcie switch that contains proprietary firmware and I think an ARM core. If you plug in a NVME board, that probably has proprietary firmware and cores. If you plug in a video card, that probably has proprietary firmware. If you plus in a usb thumb drive, that probably has proprietary firmware. The SDcard may have proprietary firmware too. Etc.
You can’t remove the pcie switch from the Unmatched board, at least not easily. You can use it without disk and video, but it won’t be very useful that way. It isn’t usable without the SDcard, and I don’t know if you can get an SDcard with free/open firmware.
The unleashed board is more open as it has fewer peripherals, so there is only the SDcard to worry about. But there weren’t very many of these made, and SiFive stopped making them two years ago, so probably not easy to find second hand.
I suppose the ftdi chip might have firmware also, that is also unavoidable, as this provides the serial console and jtag support. This affects both the unleashed and unmatched.
The pmic chip might have its own firmware too. This affects only the unmatched.
It is really hard to make a board without any proprietary firmware and SiFive has never tried. But if you only care about the firmware running on the main processor, then either unleashed or unmatched should be OK for you.