Comment by MrDOS

Yes; it means you can, for example, install an off-the-shelf copy of Debian. See https://github.com/pftf/RPi4.

In theory, it's a wonderful idea, because it means you can run the same distro on your RPis as on the rest of your fleet (architecture differences notwithstanding), and therefore have consistent patching requirements, etc. In practice, as frustrating as it is that the RPi kernel patches haven't been upstreamed, they exist for a reason and I think by not having them, you're shooting yourself in the foot (even if you only hit a toe or two). Two things I observed recently when directly comparing stock Debian aarch64 with Raspberry Pi OS on an RPi4 was a 40%+ reduction in SD card read performance, and nonfunctional power control of the USB ports.

Fwiw I still have a “poor man's NAS” RPi4 running Debian, and it works great. (The SD card throughput deficiencies don't affect me there because the system boots from a USB-attached SSD.) If you're able to take the time to make a situation-specific assessment of what works (and what works “well enough”) vs. what doesn't, then in the long term, you can reap the benefits of not having to remember how to handle a single oddball OS in your fleet (especially given RPi OS' particularly consumer-oriented whims).