They use a slightly modified Ubuntu kernel (https://github.com/proxmox/pve-kernel), with things like ZFS added. They also really are good about using proper Debian tooling, and so their kernel doesn’t cause any weird dependency issues.

Right now they install proxmox-kerne-6.8.12-6 by default (using pseudo-packages called proxmox-default-kernel and proxmox-kernel-6.8 pointing at it), and offer proxmox-kernel-6.11.0-2 as an opt-in package (by installing proxmox-kernel-6.11)

I’ve been using the latest opt-in kernels on all of my Proxmox nodes for a few years now, and I’ve never had any issues at all with that myself.