but Kbin blocks a LOT of communities that I want to interface with. I feel like I'm missing out if I'm not on Lemmy ... but I feel like I hate the user experience without the automatically hiding posts :(
this would almost certainly require an extension, since you'd need some sort of persistent way to track "viewed" items. I don't think a userscript can do it.
Best bet will probably be Instance Assistant (!instance_assistant@lemmy.ca ), at some point in the future. Could always make a request directly, too since this would likely be a relatively popular feature.
Technically correct, but you already have that extension installed if you're able to run userscripts.
The script just requires // @grant GM_setValue and // @grant GM_getValue and you can use those functions to store whatever, forever (or until you clear extension data).