We're going a bit offtopic here, but I'll answer..
Ok this is very crazy. I read your topics about coinwitness and coinconvenants twice and I didn't get it lol. I will give a better reading today as it really looks interesting.
They were particularly interesting to me because they were old articles, from2013.
Can you eli5 me about this?
If I understand correctly, after doing a soft fork nodes wouldn't check anymore if the rules were followed in your transaction, but if you did checked.
How could this lead to spending supposedly lost coins, without Private keys?
The real owner would have lost his funds , if he had the private keys and decided to spend them?
The thread is full of technical details-- so perhaps not the best summary to link from here.
You know how real property can have
deed restrictions? -- contract terms that run with the land and impose restrictions on future owners?
In theory the same could be done with Bitcoin. Where instead of just having keys and timelocks a coin could constrain have restrictions that it required you to preserve when the coin was moved. But don't worry, you couldn't accidentally accept one of these encumbered bitcoins, your wallet wouldn't recognize them unless specifically programmed to.
Currently the state of script is hobbled enough that no one knows how to do it at the moment, but it's pretty likely that it will become possible again eventually. There are really powerful and good uses for such tools, but there are also profoundly stupid ones.
So, for example (in theory) I could take a bitcoin and encumber it so that that particular coin could only ever be sent between me and you, no other key could ever control it. You and I could send it back and forth forever, if we wanted... but it would effectively be out of circulation. ... even though it was circulating. It would effectively no longer be a Bitcoin any more, but some kind of "me and you" token.
In theory, with sufficiently advanced technology, no one except the people involved with a coin could tell that it was encumbered either.
So it's possible that some day there may be circulating bitcoins that aren't really bitcoins anymore and only their owners know it.