There is a very simple solution to this: use generic mode.
Yes, that works, but some games just jump all over the place and signal gets lost. What I mean is that Saturn looks perfect and steady, but you’re bound to get problems at some point with PS1.
I can confirm NES RGB works on LG C9 without the need of a dejitter.
To be perfectly honest I only tried the NES RGB on Panasonic plasma screens, and it never worked. didn’t bother try it on the C9. The SNES however, doesn’t work at all on the C9, or on any screen that I tried. Regarding the N64, please let me know which settings you’re using, cause even in generic it looks really flickered and ugly.
But I still do wonder if there was perhaps some FW update to the LGs that made them less tolerant…
Yes, I suspect the same.