Slack is good for easy questions where you already know how to convert your thoughts into words. If you don’t because your thoughts are currently encoded visually… slack (or any online medium) doesn’t work. In the real world I would almost always abandon the slack conversation and go visit the person / people.
Email is crap for communication, at least for my working style.
The rest are okay when you’ve got a lot of clarity already but not so hot when you are very early in the ideation phases.
All of your substitutes are significantly lower bandwidth, much more latent mediums.
You can’t replace face time with online. You just can’t.