Unfortunately headphones and noise cancellation don't help with the fight vs. flight reflex that gets automatically triggered by foot traffic near your desk or especially behind your back. This is why reaching a state of flow and getting really productive is just totally impossible in an open-plan office setting period. The most basic requirement for being able to focus is privacy. Quiet is also important, but not nearly as important as privacy.

Many people also find the physical feeling of anything covering / touching / inside their ear to be too uncomfortable to focus, ruling out all possibilities of headphones, earbuds or earplugs from consideration. And even more, lots of people have sensitivity to music that distracts them or makes them feel tired very quickly. And in severe enough cases, someone can have full-blown misophonia, and truly need the ability to completely customize their workspace in order to totally control all sounds during periods when they need to focus.

I know your advice might help some people, but I really wish there was a way en masse that developers could communicate that headphones absolutely do not solve the problems created by open-plan office layouts (one of the main destroyers of focus). It's sad that as a collection of workers, we've allowed ourselves to be put into this position by not demanding private offices and turning down jobs that won't provide such a cost-effective tool.