Working on projects that have existed for years, and have been maintained by several different people, you don't always know what code there is that you could possibly get back out of the repo, without exploring it version by version.
I can imagine there being instances where leaving commented out code could be helpful -- including comments about why you thought it could be helpful, and why it is commented out!
I can almost guarantee that, if your code bases version history is too messy to find old code, then it will be even worse if you were to favor commenting out to deleting.
That said, I do see where you're coming from. Part of the issue is that we (well, most of us) don't have good ways of searching old code. There is Codeq ( https://github.com/Datomic/codeq ), which is prettydamncool™ ...hopefully we'll start to see more systems like it.
Yes, well -- and I suppose I should add that I'm not advocating for an environment in which keeping commented-out code around is a good idea. Just suggesting that there could exist situations in which it is a reasonable thing to do.
I've increasingly been noticing that a lot of really good development practices make sense if you're starting from scratch and can employ them right away, but sometimes if you've got years or decades of legacy code and legacy process (and code that was written as the result of legacy process) to deal with, the right thing to do isn't always so clear.
(I've unfortunately/fortunately been working some recently on a very large, very old code base that mostly doesn't need updating. Trying to unravel its mysteries enough to add a new feature has been an interesting experience.)
I can imagine there being instances where leaving commented out code could be helpful -- including comments about why you thought it could be helpful, and why it is commented out!