There are some estimates that between 500 000 - 1 000 000 bitcoins can be considered lost.
However, Bitcoin is still in double-digit yearly inflation, adding 3600 new Bitcoins every day to the supply, can't call it deflationary.
It's safe to assume that from now and into the future the number of lost Bitcoins will not increase very much, because in the past people didn't think much of Bitcoins and formatted drives without thinking twice.
So, let's assume 1 mln of Bitcoins will be lost out of 21 mln, that leaves 20 mln. useable max cap.
Advances in technology over time may potentially make it easier to recover some of these lost bitcoin.
As long as you have the physical hard drive that the wallet/private key was/is stored on then you potentially can recover the lost bitcoin. If you have thrown away the hard drive or the hard drive is otherwise destroyed then the bitcoin would likely be lost forever.