Target Display Mode on 2009/2010 27 iMac with half defective GPU: TDM works and boots normally with Snow Leopard

I found this article:

2009 27" Imac Bad GPU but can still use i… - Apple Community

and I tied it myself for a 2010 27 iMac with a Radeon 4850 which is defective (boots only halfway, login only with safe mode), and I can confirm that target display mode still works with those half defective gpus!

I write this, because this could be useful to people, who still use the 2009/2010 iMacs as external screens, which work on EVERY computer even Silicon Macs and PCs, because the use mini display port:

I tried something different: I installed snow leopard 10.6.6 (don't install 10.6.8 because this version breaks dimming with keyboard when in target display mode). In Snow Leopard I could still boot normally without safe mode!!!