SYMBOL is an SCP inspired supplement that adds memetic dangers to FIST, and it's straight up one of my favorite pieces of writing in FIST.
The PDF is 27 pages, with a clean, readable document-style layout. A lot of hyperlinks and references are provided to the SCP wiki, so this might also work as a jumping off point for exploring SCP if you become interested while reading.
Contents-wise, the tone of SYMBOL is fairly academic, but there is a genuinely nice content warning that takes into account that some folks might have a rougher time with memetic horror than others. There's also clear and direct communication with the GM about how best to add the material in SYMBOL into a FIST game---but it's so universal and carefully thought through that I think it'd be worth reading even if your intent is to run a different system.
SYMBOL does add a few new mechanical elements to FIST, essentially new broad saves against memetic danger like M-RESIST and C-RESIST plus a system of specific memetic immunities, and there's also (really, really good) new TRAITS and hazards, but a lot of SYMBOL's words are centered around positioning these new elements in your story. For example, the Antimemes section goes for about four pages, but there's only two TRAITS and no monsters---just instructions on what an Antimeme would look like and how it would narratively behave.
I think in another supplement this wouldn't really gel with me, but instead I kind of ended up with a brain overflowing with ideas by the time I was done reading. If you want to create SCP inspired game material, you might want to shuffle SYMBOL to the top of your to-read list.
Overall, this is an incredible (though relatively academic) supplement for not just FIST but any SCP-adjacent game. You could use the principles in SYMBOL for Delta Green, or LIMINAL, or Unknown Armies, or JoJo's. If you're a GM or a game designer that likes to work with memetic horror, I'd strongly advise giving this a read.
Minor Issues:
-Page 10, does M-RESIST work on antimemes? The section seems to imply that characters might have a RECALL score instead, or that certain experiences maybe have an innate RECALL property. However the ANTIMEMETICIST also has an M-RESIST. I couldn't quite work out authorial intent here.
-The information in some sections could benefit from being a little more centralized. M-RESIST, C-RESIST, and Inoculations could all fit in one frontloaded supersection.
-Page 23, should NAMELESS have a FORGET score?