When something is lost, it means you can not have access to it again, unless you luckily find what can grant you access to what you have lost. The possibility of losing your Bitcoin is if you no longer have access to your private key and it was an act that was very common years back, probably because Bitcoiners of that time didn't know that Bitcoin will become very appreciative to what it is today.
When something is lost, in some cases there are always chances of getting it back if there is careful retracing. The same is applicable to bitcoin.
Someone can lose access to their holdings today and still regain access to them again in times to come. Only those coins that are sent to the burn address are what I consider to be what's lost forever.