no bitcoins are ever 'lost' 'deleted' every coin still exists and is trackable on bitcoins ledger
however there are reasons where some UTXO's (unspent balances) are not moved, used, circulated between users regularly.
all bitcoin can be tracked. there is nothing secret or special. so tracking bitcoins is not a question.
the question should be judged as which bitcoins should be deemed as unusable currently, meaning what defines 'lost'
many come up with different lists such as:
UTXO's linked to public keys which have an obvious sign that the key does not have a private key someone knows of
UTXO's with dust amounts so small,, to move/spend them would cost more than the value of the UTXO
UTXO's coin-age is more than 6 year old
however even these cases are not 100% proof of those coins being 'lost'. thus when tracking the bitcoins that meet the criteria list of what most would deem as 'lost' should not be treated as a guaranteed lost value, but more of a idea/suggestive stat/estimate.
after all some UTXO's are of only 1sat. meaning people dont bother including them in spends. but in a few years if 1 sat is worth more than a chewing gum. people might want to spend it. thus bringing it back into regular use
Those are interesting facts. Knowing (or at least having an idea) of how many Bitcoins are lost, could be the key towards determining the true value of the cryptocurrency. Since Bitcoin is a fully transparent cryptocurrency, it's not that hard to trace lost Bitcoin at all. But as you've said earlier, there isn't a proof that coins are 100% lost based on said cases. For example, there could be a Bitcoin address with some balance in it that's not been touched for a long time. This could mean that the address has been used for cold storage instead of being "lost".
Still though, a mechanism that would detect lost/stolen Bitcoin would be nice, as it could return those coins to the network (which in effect, prevents them from being wasted). I believe that "Faketoshi" was working on something like this, but I can't seem to find the article related to the subject.
All in all, Bitcoin could be greatly undervalued if one takes into consideration all of the lost coins on the network.