Are there any good guesses/ assumptions, how much Monero is seized, or forever lost in a dead wallet where the seed got lost (f.e.)? One way would be a thread where people just report cases like this.

I for my part I "Destroyed" 0,34 XMR in 2017.

I forgot about this wallet when I set up a new OS (I was holding XMR only for speculation back then, I didn't know shit about Monero), and ofc I didn't write down the Seed.