It's important to point out though that if you make asking questions too easy for new employees, they can end up leaning on that as a crutch instead of developing the troubleshooting skills necessary to solve problems independently.
I've been on both sides of this fence. I've been that new colleague hunting for info that isn't clearly documented, and I've also been the senior colleague who was the go-to person whenever a problem came up that didn't have an immediately-clear answer. I was lucky in that shortly after I started my only senior resigned leaving me with nobody to ask, which forced me to get good at that kind of troubleshooting.
It makes me wonder if new colleagues should be given a time period for when they are allowed to ask any question no matter how stupid. After that time, they can of course still ask questions but it's expected that they try to find an answer before asking. Along the same lines of pair-programming for colleagues who have been here less than 1 year.
Their opinions matter too. Junior developers will probably suffer without easy access to those around them (yes, it's annoying for the senior devs, but that's part of the job). PM's, similarly, need to be effective; in a well-run company, they're there to help and if open-offices are important to that, well, there you go.
I'm a fan of slack etc, but I'm also an extravert. If I'm not talking to others as a part of my job, I'm going to be miserable and leave. I don't think I'm the only one.