18.04.2021, 23:33
I tested the dev version and tried all the Vapoursynth deinterlace filters. All of them work! I finally managed to make the QTGMC deinterlaced output and hopefully be able to process my home video library after years of keeping it in a drawer. 
Selur, I really appreciate your support, thank you for it!
BTW: Does the OpenCl support work for AMD card in Hybrid on Linux (Ubuntu)? I tried to turn it on for the x264 codec, but did not realize any difference in CPU utilization or encoding speed. When I turned it on for QTGMC deinterlace filter, the output generation failed. I have OpenCl working and use it in darktable photo editor. I just wonder if I am not missing anything obvious. I did search the forum, bud did not find anything useful for my case. Although not happy, I will understand if the answer will be "do forget on OpenCl support for AMD"...
Selur, I really appreciate your support, thank you for it!
BTW: Does the OpenCl support work for AMD card in Hybrid on Linux (Ubuntu)? I tried to turn it on for the x264 codec, but did not realize any difference in CPU utilization or encoding speed. When I turned it on for QTGMC deinterlace filter, the output generation failed. I have OpenCl working and use it in darktable photo editor. I just wonder if I am not missing anything obvious. I did search the forum, bud did not find anything useful for my case. Although not happy, I will understand if the answer will be "do forget on OpenCl support for AMD"...