Hi, I have a problem with bit depth. I want to create an image with a depth of 8-bit. It is required for one old pc game.

Krita apparently have a bit depth associated with channels and the minimum depth is 8-bit per channel and by default, there are 4 channels. I found in documentation following:
8bit per channel
Also known as “True Color”, “Millions of colors” or “24bit/32bit”. The standard for many screens, and the lowest bit-depth Krita can handle.

My question: Is there a way of achieving real 8-bit depth with some trick? Maybe some one channel setting or something. Just curious.

Thanks and happy holiday

No, sorry, that's not possible, and it will not be possible. There are filters that will limit the number of colors used, but your image will still be RGBA. GIMP has an indexed color mode, so maybe you should try using that application, which is also free software.