Comment 6 for bug 1765556

You should read the changelog.

nvidia-prime doesn't use bbswitch anymore, I don't know why. The changelog doesn't say.

It now uses the built-in kernel mechanism for turning off the nvidia card. Unfortunately, this method requires the nouveau driver and unfortunately you can't load the nouveau driver and the nvidia driver at the same time. Also, unfortunately the nvidia driver can't be unloaded without a reboot, due to a bug in logind, or at least without restarting logind.
Somehow, this also means that the nvidia driver needs to be actually uninstalled, which means rebuilding initramfs to uninstall nvidia. And going back to nvidia (hybrid mode) now requires the nvidia driver to be reinstalled. It's a far cry from the old way.

It's a bit of puzzle because bbswitch still works in the latest kernel and with the latest nvidia. But prime select is pretty complex, I haven't found another distribution where it has been ported, so there are probably good reasons for this. The dev says this approach is needed to work around gdm3 bugs, which I find ironic since I have never got gdm3 working with my nvidia laptops anyway (I use modeset=1 with the nvidia driver).