I'm most curious to know how one cracks a word-based substitution without knowing the key.

I have 175 hours on BOTW and 800 hours on Splatoon 2.

I'm being somewhat optimistic when I say a few years, but realistically it will probably be another decade.

It sounds like you are describing ghosting which is normal. I have an LG E6 (2016) so it doesn't have the anti burn-in features that the newer ones have. I had it on 100% OLED brightness up until last year when I started noticing the degradation, so now I keep it at 80%. It now has burn-in showing BOTW's hearts in the top left, Splatoon 2's matchmaking screen on the right half, and some strange perfectly vertical bar down the center that I can't explain. I've played both a decent amount but not anything that would be considered abnormal or excessive.

Edit: I just remembered I had posted a picture of it on reddit last year:

The Splatoon matchmaking screen (and other Splatoony stuff) is burned into my OLED TV

Agreed. The only stuff burned into my OLED TV right now are UIs from a couple video games. Fortunately I don't notice them unless there are solid colors being displayed.

OLED is far superior to anything else out there right now but burn-in continues to be a concern. My next TV (probably in a few years) will hopefully be a Micro LED which has all of the advantages of OLED and none of the downsides.