Many companies in my experience do test for these things. They remove old features and measure impact periodically, or run a long term hold out bucket, or some

Many companies in my experience do test for these things. They remove old features and measure impact periodically, or run a long term hold out bucket, or some other such approach. The deep dark secret of the web is that there often there isn't a negative impact that can be measured no matter how much people try to measure one.