I wonder if that can be verified
What do you mean? Verified mathematically if the old wallet mining would be event possible?
I also asked on #StackerNews and got into a new rabbit hole https://stacker.news/items/583764/r/alashazam there are 'burned coins' and lost wallets but the lost bitcoins are mostly the first category (the second type are still valid and we may assume that someday someone will access the lost wallet) Hope it make sense (I am still very new to that stuff, and get easily confused)
The amount of unclaimed rewards (burned coins?) should be verifiable on the chain, right? The rest exist the same way as a Roman gold coin exists few meters deep in a potato field somewhere in the middle of nowhere... It can be recovered tomorrow, in 25 years, in 250 years or never.