ADMIN EDIT

First Raspberry Pi 5 testing images are available now on our download page: https://dietpi.com/#download
You can also find images with the new kernel/firmware package set for other RPi models here: https://dietpi.com/downloads/images/testing/
The ones with the new firmware have "RPi1", "RPi2" and "RPi234" (64-bit) in their names. To migrate an existing system, use the migration script:

bash <(curl -sSf 'https://raw.githubusercontent.com/MichaIng/DietPi/dev/.meta/dietpi-rpi-firmware-migration')

Update: The script has been replaced with a dietpi-config option:

• dietpi-config > Advanced Options > RPi firmware migration

WARNING: This is a one-way ticket. dietpi-backup cannot be used to restore the old system, since the boot partition mount point has changed. If you want to be able to revert, create an image of the whole SD card/drive. dietpi-imager can be used from another DietPi (or Debian/Ubuntu) system, to minimise partition and filesystem automatically, to have a small compressed backup image.

• Device name | Raspberry Pi 5
• Official product URL | https://www.raspberrypi.com/products/raspberry-pi-5/
• GitHub resource URL (if available) | https://github.com/raspberrypi
• Image download URL |https://www.raspberrypi.com/software/

Is the SBC officially supported by the Debian installer?

• Yes(?) Looks like the board is inter-compatible with the same distro images as the 4B. That's pure speculation, mind you, but I did see a video of a reviewer transferring a Raspbian installation from a Pi4B to a Pi5 with no configuration necessary.

Notes

• Not sure if this needs any specific developer legwork as the previous Pis seem to be inter-compatible with OS images for the most part (though I'm sure testing and drivers will be necessary!). Thought I'd make this issue now since it might be good to mark the Pi5 as provisionally supported on the website if it at least works out of the box.