This is an issue that has plagued the game since release with the upscalers and I've seen little mention of it online.

Shadows, when set to High or Very High will cause severe flickering on some surfaces and areas in the game (though mainly on clothing and weapon pieces) when one of the upscaler options is enabled. I have even seen some instances of the flickering with Medium shadows but it's largely more rare and mitigated.

Here is a video I uploaded showcasing the issue:
https://youtu.be/h9V7VJdZTv0

It's been a while since the last patch was released but maybe if we can get more support tickets sent about this bug it will force them to investigate and fix it.

Here is the support link: https://www.playstation.com/en-ca/horizon/support/

Click on Forbidden West, then "Technical (Performance, Crashes)" then "Other" twice until you get to the page to fill out a ticket. Don't forget to put "PC" as the platform.

To easily see the issue, go to the tavern in Chainscrape and initiate dialogue with Petra, you'll see the flickering like in the video with DLSS or FSR enabled as well as shadows set to High or Very High.