Karabiner for Big Sur is available here:
There’s one caveat:
macOS Big Sur Beta 6 has a very serious DriverKit issue.
Karabiner-Elements will stop working after restarting your Mac due to the issue.
You have to reinstall the driver by the following way every reboot on macOS Big Sur Beta 6.
“Reinstall” is a strong way to put it. When you reboot, you have to click a button, enter your macOS admin password, and then click another button. That’s it!