How much digital currency is lost forever?

Good question. And this may be one of the biggest problems with cryptocurrencies, both in terms acceptance as legal tender and attracting investors.

At least once source estimates key-related crypto losses average about 20% :
http://www.btcethereum.com/blog/2021/03/16/how-much-crypto-has-been-lost-due-to-assets-being-sent-to-the-wrong-addresses/

Another online analyst put total losses for 2021 at $150 bil so far : https://techjury.net/blog/cryptocurrency-statistics/

I have no way to disprove or argue with @JLoon ‘s research.

My guess is – a lot – because digital currencies are unregulated, there is no transparent bookkeeping, and people who lose money don’t advertise it.

My reaction – digital currency is an artifical construct – a Potemkin village. Nothing real about it except expectations. In that respect, it is another false religion.

Well, it’s real enough to trade for traditional money, and to buy and sell services with, at least with some/many people. It’s rather more volatile than traditional money, at least in some ways, as far as we know now.

You can always blame Sheldon.