So I put together a "private mode" chrome extension, which simply conditionally blocks certain requests to Youtube.

I "found out" which requests by simply looking at all the requests that get fired, and figuring out which one's might be related to recommendations.

I tested this after building by using one a Youtube account and watching a video that's completely out of it's current world of recommendations with and then without private mode enabled to confirm it does what it should.

Current limitation is that it only blocks videos from impacting your recommendations, not searches.

From what I see searches only influences recommendations slightly, but it still does.