Zaerc

Addendum: How To Include Devicetree Overlays
Since Grub does not have devicetree overlay support, we need to merge the desired overlay(s) in with the main devicetree (.dtb) file we're using.
 
Part of the device-tree-compiler package is this command:
fdtoverlay
 
An example from Armbian 25.8.1 Trixie running on the Orange Pi 5 Plus, to enable GPU acceleration in the Vendor (6.1) kernel. In sudo:
cd /boot/dtb-6.1.115-vendor-rk35xx/rockchip fdtoverlay -i rk3588-orangepi-5-plus.dtb -o rk3588-orangepi-5-plus-panthor.dtb overlay/rockchip-rk3588-panthor-gpu.dtbo  
Of course, to have that load automatically from grub using the modification above, you'll need to rename (or move) the original .dtb to something else, then rename the one with the -panthor suffix to the original name to take its place.

I also upgraded from bookworm to trixie and saw this warning. I can confirm that Igor's suggested workaround addresses it:
wget https://apt.armbian.com/armbian.key gpg --dearmor < armbian.key | sudo tee /usr/share/keyrings/armbian.gpg > /dev/null apt is now happy with the armbian repo.