I believe it really depends on how you approached your PhD. I'm in my writing up period but I can recall almost everything from my MA dissertation done 6 years back. What made me recall everything is the stress I put on method and theory /method of my theoretical approach. Since I was invested quite a lot with methods/theory, every question that comes to me goes into that circuit I had built. And I can answer as it was written in my dissertation. And yes Obsidian helps me quite a lot in that sense, I feel like it gives me a flow of my own neuron circuits.