read somewhere that 20% (give or take) of all bitcoin been already lost.
That is wrong. People have come up with their guesses but they are either based on their opinion or very little research simply because we have no true way of measuring how much bitcoin is lost. Not to mention that usually these people who come up with such percentages use the "coins not moved for X days == lost" logic which makes no sense at all.
As bitcoin becomes more mainstream, is it reasonable to expect that more and more bitcoins will be lost?
No, because as bitcoin's adoption grows its price also goes up and that means 2 things. First people can acquire less bitcoin (for example you could buy a thousand BTC when it was worth less than a US cent but you can't buy 0.01 BTC now that it is almost $20k as easily) so if they lose it they will lose smaller amounts and Second since bitcoin is much more valuable now they will take a lot more care to not lose it.
This means as adoption grows less and less bitcoins will be lost.