I think you'll need to reboot for the microcode to be updated

Probably eventually yes, but if you are really concerned you need to discuss it with your distro maintainers.

This. Not everyone is as quick as say Arch or Fedora in updating/patching. Please reach out to your maintainers of the distro you use.

Even Arch seems out of date as of 24 jul 2023 17:55 UTC.

The latest amd firmware version is 20230625.

Gentoo already has it, however the latest ebuild is still masked, so one would need to put "sys-kernel/linux-firmware ~amd64" inside a file in /etc/portage/package.accept_keywords, or better yet, always run the git version, using * instead of ~amd64.

Apart from that, it's necessary to "sudo emaint sync -A && sudo emerge -av sys-kernel/linux-firmware", while checking that the correct files are included in the savedconfig file if using it. After that, rebuild the kernel or the initramfs and reboot.

The 6.4.6 kernel has mitigations, but the arch “linux” package is still at 6.4.5.