To chime in regarding my experience with image retention: I've been playing games with huds (ie; including bright red health bars such as Withcer 3) ever since I got my B6 OLED.. I've gone up and made lunch, played for 8 hours straight, etc.. Still not a smidgen of image retention.
The screen auto-dims after not using for about 5 minutes and there's an 'auto wipe' to shift the OLED pixels to reduce the chance of image retention, with these two features in place it seems one would be hard-pressed to run in to this issue (with the LG B6 anyway, can't say for certain about the 'features' to mitigate this for other panels).
Not saying it's not possible, but I've done heavy use with it ranging from watching YouTube videos, Twitch Speedrunners (which have livesliplit running which is a static sidebar with often bright colors), Video games with bright hubs, Movies with black bars, etc..
Still 100% perfect panel to date.